A 2026 CIArb report on party-appointed expert witnesses in international arbitration suggests that experts are often brought into disputes later than they should be. The findings are about arbitration, but many of the issues will be familiar to those involved in conventional litigation as well.

Party-Appointed Expert Witnesses in International Arbitration: Effectiveness and Best Practice draws on 419 responses from expert witnesses, arbitrators, external counsel and in-house counsel, together with consultation across common law and civil law jurisdictions. One of its central themes is the timing of expert involvement. Some 71% of respondents thought that, ideally, experts should be appointed before the arbitration itself had begun: 42% once the issues between the parties were known or had crystallised and 29% before a claim was made.

The figures become more interesting when the responses of experts are compared with those of the people instructing them. Only 44% of expert witnesses thought they were typically given enough time to carry out their work. External counsel were considerably more optimistic at 66%, arbitrators at 77% and in-house counsel at 80%. Some 60% of the experts also thought they should have an opportunity to comment directly on the procedural timetable when it is being set.

There was a similar difference of view about why experts are instructed late. Among experts, 42% identified a lack of understanding of the expert’s role and 39% identified cost. External counsel saw cost as the principal problem, at 53%, while only 13% thought lack of understanding of the expert’s role was the main reason.

Those figures come from international arbitration, but the underlying question is wider: are we sometimes concentrating too much on the date by which the expert report is needed, rather than the point at which the expert might usefully become involved?

What can an expert add before the report is due?

Bringing an expert in earlier does not necessarily mean commissioning a full report earlier. There is useful work which may need to happen long before anybody starts drafting one.

An expert may be able to identify whether the technical question being posed is actually the right one, what evidence or data will be needed to answer it, whether important material is missing and whether the proposed timetable bears any relation to the work required. In some disputes the expert may also be able to test whether a claim or defence which looks attractive in legal terms works when the underlying technical, scientific or financial assumptions are examined.

The CIArb report identifies early involvement as potentially helping to test the viability of claims, clarify technical issues, narrow areas of disagreement and inform procedural planning. It also raises the practical difficulty of experts being presented with a timetable once it has already been agreed and then being expected to work within it.

This can become particularly important where several disciplines depend upon one another. A quantum expert, for example, may need findings or opinions from an engineering or delay expert before parts of the quantum analysis can sensibly be completed. If those dependencies are only discovered after individual report deadlines have been fixed, somebody is likely either to duplicate work or to run out of time.

The same issue arises in litigation

In England and Wales, CPR Part 35 requires a party seeking permission for expert evidence to identify the field in which the evidence is required and the issues which it will address. Where practicable, the proposed expert must also be identified. Practice Direction 35 separately requires parties to consider with their experts at an early stage whether an experts’ discussion is likely to serve a useful purpose and, if so, when it should take place.

By that stage some fairly important decisions may already be taking shape. What is the expert issue? Is one discipline enough? Will more than one expert be needed? What material will each expert require? Is sequential evidence sensible? Should the experts meet before or after reports?

Our own Guidance on Joint Statements reflects the fact that there is no single correct sequence. In some cases an early meeting between experts can expose differences in methodology, assumptions or source material before substantial costs are incurred. In others, particularly less complex cases, exchanging reports first may lead to a more focused discussion.

The appropriate sequence depends on the dispute and the evidence. What matters is that it is considered rather than allowed to happen by accident.

Different approaches in different jurisdictions

There is no universal model for expert evidence. This is particularly relevant in international arbitration, where participants may come from very different procedural traditions and the arbitration itself may combine elements which would be unfamiliar in the domestic courts of any one jurisdiction.

Australia provides an interesting comparison. The Federal Court expects parties proposing to use expert evidence to consider at an early stage how that evidence should be managed. Its Expert Evidence Practice Note deals with matters including the questions to be addressed by experts, conferences between experts, joint reports and concurrent expert evidence.

Australia is particularly associated with concurrent expert evidence, sometimes known as the “hot tub”, under which experts may give evidence together and address particular issues in turn. That can make expert involvement part of the management and narrowing of the technical dispute rather than simply a sequence of reports followed by separate cross-examinations.

Singapore takes a rather controlled approach to the use of expert evidence. Under the Rules of Court 2021, expert evidence cannot be used without the Court’s approval. The parties must consider whether it will materially contribute to deciding an issue involving scientific, technical or other specialised knowledge and whether the issue could instead be resolved through agreed facts or agreed materials.

The Singapore courts also require the parties, as far as possible, to agree on a common expert, and the issues to be referred for expert evidence and the agreed or assumed facts are themselves subject to close control. The Singapore Judiciary provides a useful summary of the expert evidence provisions.

Hong Kong also requires permission before expert evidence is adduced in civil proceedings. An application is normally made after pleadings have closed and should identify the area of expertise and the issues on which expert evidence is required. Parties are also expected to consider whether a single joint expert would be appropriate. The Judiciary summarises the process in its guidance on preparing for a hearing or trial.

In US federal litigation, the structure is different again. Rule 26 of the Federal Rules of Civil Procedure requires disclosure of testifying experts and, for retained experts, detailed written reports. The timing is normally set by the court; if it is not, the default deadline is at least 90 days before trial, with a shorter period for rebuttal evidence. That is a disclosure deadline rather than a rule about when the expert must first be retained, and experts in substantial cases may of course be involved considerably earlier.

These differences make “instruct the expert earlier” rather too simple as a universal rule. Some systems favour early involvement of party-appointed experts; others put more emphasis on defining the expert issues first, limiting the amount of expert evidence or considering a common or single joint expert.

The point which travels rather better between jurisdictions is that expert evidence should be considered early enough for the procedure to fit the evidence, rather than forcing the evidence into a procedure which has already been fixed.

Earlier involvement does not mean losing independence

There is an obvious concern in bringing an expert into a dispute at an early stage. A party-appointed expert witness is not another member of the legal team and should not become an advocate simply because they have been present while the case is developing.

But there is a difference between asking an expert to construct the client’s case and asking the expert whether the case being contemplated can be supported within their field of expertise.

An expert may quite properly say that a proposed methodology will not work, that the available data is inadequate, that another discipline is required or that an assumption on which the claim is being built is technically unsustainable. Finding that out before pleadings are settled and directions are agreed may be considerably more useful than discovering it three weeks before the expert report is due.

The capacity in which the expert is instructed also matters. An expert retained privately to advise a legal team performs a different function from an expert who is to give independent evidence to a court or tribunal. The distinction between advisory or “shadow” experts, party-appointed expert witnesses and single or common experts deserves separate treatment, particularly because duties, disclosure and privilege can differ between jurisdictions.

Start with the expert evidence, not just the expert report

The CIArb research does not suggest that every expert should be appointed at the beginning of every dispute. The appropriate approach will depend on the dispute, the tribunal or court, the jurisdiction and the type of expert evidence required.

It does suggest that leaving consideration of expert evidence until the case has already been designed can create problems which are difficult to repair later. Timetables may prove unrealistic, necessary evidence may not have been preserved or obtained, different expert disciplines may have been sequenced badly and the questions eventually put to the expert may not be the questions which actually need answering.

That should sound familiar outside arbitration as well.

When a dispute is likely to depend materially on expert evidence, the useful question may not be simply “When do we need the report?”

It may be worth asking rather earlier: “When do we need the expert?”

Further reading: CIArb, Party-Appointed Expert Witnesses in International Arbitration: Effectiveness and Best Practice.

An expert may have sound reasons for favouring a particular answer. But explaining those reasons is only part of the task. Where there are competing opinions, the report also needs to show that they have been considered fairly, including the evidence which does not sit comfortably with the expert’s own conclusion.

JBX v Frimley Health NHS Foundation Trust [2026] EWHC 2294 (KB), decided on 4th September, 2026 by Andrew Kinnier KC, sitting as a Deputy Judge of the High Court, concerned damages following admitted clinical negligence. One issue was future care and accommodation for a profoundly brain-injured claimant.

What the court found

The judge identified two particular concerns about the claimant’s neuropsychiatric evidence:

• The expert favoured home care but had inadequately examined its disadvantages and the advantages of residential care.
• The judge criticised his persistence with a diagnostic opinion outside his expertise.

As a result, the defendant’s evidence was preferred on disputed neuropsychiatric matters (paragraphs 95–97).

Similar problems affected the claimant’s care evidence.

• A letter acknowledged “positives and negatives” but listed only home care’s benefits and residential care’s drawbacks.
• The judge found the overall approach one-sided (paragraphs 113–114).

Both care experts were giving oral evidence for the first time. Despite the defendant’s expert’s marked nervousness, her report assessed competing options with reference to medical evidence. Her evidence was preferred where they disagreed (paragraphs 112–117).

The reporting requirement

In civil proceedings in England and Wales, Practice Direction 35, paragraph 3.2(6), requires an expert’s report to summarise the range of opinion, where one exists, and give reasons for the expert’s own opinion. Paragraph 2.3 requires consideration of material facts which might detract from that opinion, while paragraph 2.2 requires objective, unbiased assistance rather than advocacy.

There is a practical difference between those requirements. An expert might describe several professional approaches yet omit an inconvenient fact. Equally, a report might accurately record the facts but give no meaningful account of why another expert could interpret them differently. Both the evidential material and the competing analysis need attention.

This sits within the overriding duty under CPR 35.3 to assist the court on matters within the expert’s expertise. That duty takes precedence over obligations to the person instructing or paying the expert. A useful way of approaching the report is therefore to ask what the court needs to understand, rather than what makes the instructing party’s position easiest to defend.

Considering an alternative does not mean endorsing it

For example, suppose a building expert recommends replacing a defective component rather than repairing it. The report could explain why replacement would solve the problem, then dismiss repair as inadequate. A more useful comparison would identify what the proposed repair could achieve, the assumptions on which it depends and the evidence that causes the expert to reject it. The disadvantages of replacement, such as disruption or unnecessary intervention, would also need examination where they matter to the issue being addressed.

That exercise might leave the opinion entirely unchanged. It would nevertheless give the reader a much clearer explanation of why it was made. Alternatively, it might reveal that a repair is viable in circumstances the expert had not initially considered, or that further investigation is needed before choosing between the options.

Experts must, therefore, be equally clear about the question they are answering. “I would choose X” and “Y is not reasonable” are different propositions. Explaining a preference does not, without further analysis, establish that another approach falls outside a reasonable range.

This is not an argument for artificial balance or equal space for every suggestion. A plainly inapplicable alternative may need only a short explanation. A credible competing approach to a central issue deserves more. As a practical drafting test, consider whether a reader could understand the strongest argument against your conclusion without having to reconstruct it from the other side’s report.

Make the reasoning available to the reader

The Academy’s Model Form of Expert Report distinguishes facts, assumptions and opinions, including opinions taken from other experts. It also emphasises linking conclusions to the supporting text. Those distinctions help readers follow the reasoning rather than simply being asked to accept the author’s assurance that the necessary work has been done.

In practice, it is worth checking the route from each important conclusion back to its supporting material.

• Which observation, calculation or professional judgment carries the conclusion?
• What would change if an assumption proved wrong?
• Does the report explain why apparently contrary material does not alter the answer?

An appendix can contain the underlying detail, but the reader still needs to be shown how that detail affects the opinion.

Care is also needed where the reasoning depends on another discipline. The Model Form distinguishes between opinions of others on which the writer relies in forming their own opinion and those the writer accepts but cannot comment on authoritatively. A clear report should make that dependence visible rather than allowing the reader to assume that every underlying conclusion has been independently assessed by its author.

Carry the analysis into the joint statement

The same care is needed when the experts discuss their differences. CPR 35.12 provides for court-directed discussions and a statement recording agreement and disagreement, with reasons for the latter. The Academy’s guidance on joint statements addresses how those issues and reasons can be presented usefully.

A statement that the experts continue to disagree adds little unless it explains why. The difference may concern an assumed fact, the suitability of a method or the weight attached to particular evidence. Identifying that difference precisely is more helpful than placing two firm conclusions alongside one another. It also gives each expert an opportunity to check whether the disagreement is as extensive as it first appeared.

Before signing a report or joint statement, try reading it from the position of someone who favours the alternative. Have their substantive reasons been addressed, or merely their conclusion rejected? The aim is not to make the expert less decisive, but to make the basis of the decision open to proper examination.

These aspects of presenting and evaluating expert evidence are also covered in the Academy’s The Expert’s Report training.

Read the full judgment in JBX v Frimley Health NHS Foundation Trust. The assessments of the neuropsychiatric and care evidence are at paragraphs 95–97 and 112–117 respectively.

An expert is perfectly entitled to reach a strong opinion which supports the case of the party instructing them. The difficulty is when the reasoning appears to work the other way round, with the party’s case driving the expert opinion.

That distinction arose in KS v Secretary of State for the Home Department, where First-tier Tribunal Judge Froom considered expert evidence about conditions in Tajikistan. His criticisms ranged from whether the expert had demonstrated the necessary expertise through to the sources relied upon and the extent to which the appellant’s account had been independently examined. By the end of that analysis, the Judge’s view was that the expert was advocating for the appellant rather than providing critical and objective expert evidence.

The limits of an expert’s expertise

The expert described himself as a publicist, PR expert, television presenter and journalist specialising in Russia and post-Soviet states. He held degrees in international law and marketing and had written extensively for newspapers and websites.

The Tribunal was concerned about how much of that experience demonstrated expertise specifically in Tajikistan. Judge Froom noted that the expert did not appear to have visited the country, had held no academic post and listed only one publication, which concerned Russia. He concluded that sufficient expertise on Tajikistan had not been demonstrated.

This is an issue which can easily arise where someone has genuine expertise across a broad subject or geographical area. The question is not simply whether an expert is knowledgeable, but whether their knowledge and experience extend far enough to support the particular opinions they have been asked to give.

Checking the sources

Some of the most striking criticism concerned the source material.

The report stated that honour crimes were deeply ingrained in Tajik culture, particularly in rural areas, and cited four sources. Two dealt with domestic violence against women and the Tajik government’s response, another with religious freedom and the fourth was the Criminal Code of Tajikistan.

“I was not able to find any reference to honour crimes in any of these documents.”

Counsel was invited to identify such a reference and was also unable to do so. Judge Froom described the failure accurately to source the assertions about honour crimes as a “serious flaw in the report”.

The question of sources also arose in SU (Expert reports – format) Bangladesh [2026] UKUT 317 (IAC), which we considered in our article Online sources are not the expert opinion.

In SU, the Upper Tribunal emphasised that material relied upon should be identifiable and capable of being checked, and that experts should go back to the primary source wherever possible. The report must also distinguish between the underlying material and the expert’s own analysis and opinion.

KS illustrates the problem from a slightly different direction. Identifying a source is not enough if the source does not actually support the proposition for which it is being cited. The two decisions therefore fit rather neatly together. In SU, the emphasis was on making the source material transparent and traceable; in KS, the Tribunal was able to trace the sources and found that they did not support an important assertion in the report.

For experts who rely on academic papers, technical literature, industry publications, statistics or other external material, the practical discipline is much the same. The reader should be able to identify what has been relied upon, find it if necessary and understand what the expert says it establishes. A citation does not acquire authority merely by appearing in an expert report.

Assumptions and the client’s account

The Judge was also concerned about the way in which the expert dealt with information provided by the appellant.

Claims had been made about the influence of members of a particular family. The expert did not say that he had investigated those individuals or independently established the extent of their influence. Judge Froom said that the expert:

“simply accepts what the Appellant says and agrees with him”

and concluded that this did not assist the Tribunal.

Expert reports will often depend upon factual assumptions supplied by those instructing the expert. There is nothing unusual about that, and in many cases the expert will have no means of establishing those facts independently. What matters is being clear about the distinction between an assumed fact and something the expert has independently established, and not presenting the former as though it were the latter.

There is an interesting contrast elsewhere in the judgment. Judge Froom identified an issue concerning the alleged police search on which expert evidence might have assisted the Tribunal, but the report did not address it. The report therefore ventured into areas which concerned the Tribunal while leaving unanswered a question on which the expert might have been able to provide useful assistance.

When opinion becomes advocacy

Against that background, Judge Froom said:

“The distinct impression gained is that Mr Kharitonov is simply advocating on behalf of the Appellant”

rather than providing a critical and objective expert opinion. He went on to say that the expert “trespasses into the role reserved to the Tribunal”.

An expert does not preserve their independence by avoiding firm conclusions or carefully balancing every opinion. If the evidence leads strongly in one direction, the expert should say so. Independence lies in the process by which the opinion is reached: considering the relevant material, testing assumptions where appropriate, acknowledging the limits of the expert’s knowledge and allowing the evidence to determine the conclusion.

Seen together, the criticisms in KS explain why the Tribunal became concerned about advocacy. Questions about the expert’s specific expertise would not necessarily lead to that conclusion on their own. Nor would one poorly chosen citation. But where important assertions are unsupported by the sources cited, the client’s account is accepted without sufficient examination and opinions move into matters for the Tribunal, the cumulative effect can be rather different.

Judge Froom attached no weight to the report as expert evidence.

For anyone preparing an expert report, KS is a useful illustration of why independence has to be apparent throughout the report. It can be seen in the treatment of instructions and assumptions, in the selection and checking of sources, in recognition of the limits of the expert’s own expertise and, above all, in whether the reasoning genuinely leads to the opinion rather than being used to support a conclusion already reached.

Read the judgment: KS v Secretary of State for the Home Department on Find Case Law.

A recent Upper Tribunal decision gives some unusually practical guidance for experts who rely on material found online.

In SU (Expert reports – format) Bangladesh [2026] UKUT 317 (IAC), the Tribunal considered expert reports which drew heavily on publicly available internet material. The decision is particularly useful because it deals with something that now affects experts in many fields: how to use online source material properly in an expert report.

The Tribunal’s guidance is fairly straightforward, but important.

Where an expert relies on material found online, the source should be identifiable and capable of being checked. So far as possible, the expert should use the primary source rather than a secondary report about it. If material is quoted, it should be clearly identified as a quotation, with a link to the source and a translation where appropriate.

That is the sourcing side of the exercise.

But there is a separate point which is perhaps more important.

The material found online is not the expert opinion.

It is source material from which the expert may form an opinion.

The expert’s task is to consider that material, explain its relevance and significance, and then form and express an independent opinion based on it.

Both parts matter.

The underlying source material must be sufficiently transparent for the court or tribunal to understand where it came from and, where necessary, check it. But the report must also show the expert’s reasoning. The court needs to be able to understand how the expert moved from the underlying facts and sources to the opinion ultimately expressed.

It is then for the court or tribunal to decide what weight to give that opinion.

Primary sources matter

One point made expressly by the Upper Tribunal is that experts should, where possible, go back to the original source.

That may sound obvious, but it is increasingly easy to rely on a newspaper article, online summary, social media post or another secondary source which itself refers to an underlying document.

For an expert, that can create unnecessary uncertainty.

If a government statement, NGO report, technical paper, regulatory document or other primary material exists, there is obvious value in identifying and considering that material directly.

The Tribunal also stressed the importance of making quotation clear. Material copied from elsewhere should not appear in the report as though it were the expert’s own words.

That is not simply a matter of presentation. It enables the reader to distinguish between:

• the underlying source;
• the facts or information derived from it;
• the expert’s analysis of that information; and
• the expert opinion.

Those distinctions are central to a properly reasoned expert report.

The Academy’s own guidance on the Model Form of Expert Report reflects the same need to distinguish clearly between matters of fact and opinion.

Applying the material to the case

The Tribunal also made clear that a report should do more than set out general material.

Where the expert has been asked to consider the particular circumstances of a party, the report should apply the expert’s general conclusions to those particular facts.

That is an important part of what makes the evidence expert evidence rather than simply research.

A court can often read source material for itself. The value of the expert lies in the specialist analysis and opinion brought to that material.

That may involve explaining technical significance, identifying limitations, placing information in context, drawing on professional experience or explaining how apparently conflicting material should be understood.

The Academy’s What is an Expert Report? guidance similarly stresses that an expert report is not merely a statement of conclusions. The factual basis and reasoning leading to the opinion matter.

Responding to criticism

There is another useful practical point in SU.

Where one of the parties has raised concerns about an aspect of the expert report, the Tribunal said the expert should address those concerns directly, bearing in mind the overriding duty to assist the Tribunal on matters within the expert’s expertise.

That matters because an expert’s duty is not to defend the report at all costs.

If a legitimate criticism identifies an error, omission, weak source or assumption which needs reconsideration, the expert should deal with it.

The Academy’s Code of Practice for Experts places independence, impartiality, objectivity and integrity at the heart of expert practice.

A practical consequence of the online world

The volume of information available to experts has changed enormously.

In many fields, an expert can now locate hundreds of sources in a relatively short period. That can be extremely useful, but volume is not the same as quality.

The discipline lies in showing:

• what material has actually been relied upon;
• where it came from;
• why it is relevant;
• what the expert makes of it; and
• how it contributes to the opinion expressed.

That is why traceability matters.

The court or tribunal does not simply need an expert who can find information. It needs an expert who can analyse that information, explain it and give a properly supported opinion within their expertise.

That is the useful lesson from SU (Expert reports – format) Bangladesh.

Decision:
https://tribunalsdecisions.service.gov.uk/utiac/2026-ukut-00317

BAILII:
https://www.bailii.org/uk/cases/UKUT/IAC/2026/317.html

Related Academy resources

• Model Form of Expert Report
• What is an Expert Report?
• Code of Practice for Experts
• The Expert’s Report training

The High Court’s judgment in Various Claimants v Mercedes-Benz Group AG and others [2026] EWHC 1753 (KB) is a useful reminder that technical expertise, however impressive, is only part of the job of an expert witness.

The litigation involved a huge amount of highly technical expert evidence across a number of disciplines. What is particularly interesting is the contrast in the judge’s assessment of the experts themselves.

Some were praised for the quality and fairness of their evidence. Others attracted criticism over aspects of their compliance with Part 35.

That contrast matters because the experts involved were not lacking in technical expertise. They were experienced specialists dealing with complex issues in major litigation.

The point is rather that being an expert in your field and being an effective expert witness are not quite the same thing.

Under CPR Part 35, an expert’s role is to assist the court on matters within their expertise. That duty overrides any obligation to the person instructing or paying them.

Practice Direction 35 goes further. Expert evidence should be the independent product of the expert, uninfluenced by the pressures of litigation, and experts should consider all material facts, including those which might detract from their opinions.

Those requirements affect far more than the declaration at the end of a report.

They affect how the expert approaches the evidence in the first place.

An expert needs to be able to distinguish between what they know, what they have been told and what they have inferred. They need to consider material which may not support the position of the party instructing them. They need to explain the basis for their conclusions and, where appropriate, acknowledge that there may be a legitimate range of professional opinion.

Perhaps most importantly, they need to remain willing to reconsider their conclusions as the evidence develops.

That may sound elementary to experienced expert witnesses. But the significance of the Mercedes-Benz judgment is that these issues arose in extremely sophisticated litigation involving highly experienced specialists.

The technical complexity of the subject matter did not lessen the importance of the expert’s procedural and professional duties.

If anything, it made them more important.

Where the underlying issues are difficult for a court to evaluate without specialist assistance, the court is particularly dependent upon the expert presenting that specialist knowledge independently, fairly and in a way that can be understood and tested.

That is why the quality of expert evidence cannot be measured simply by the expert’s qualifications or standing in their profession.

The court also needs to be able to trust the process by which the opinion has been reached.

The Academy’s guidance on party-appointed experts reflects this. The expert’s role is to provide unbiased opinion within their expertise and not to assume the role of advocate.

The Academy’s Code of Practice for Experts similarly places independence, impartiality, objectivity and integrity at the centre of expert practice.

And because much of the expert’s evidence will ultimately be contained in the report, the Academy’s Model Form of Expert Report is designed to help experts present their reasoning in a form which assists the court. The Academy’s The Expert’s Report training also deals specifically with compliance, professional guidance and the practical preparation of expert evidence.

The broader lesson from the Mercedes-Benz litigation is fairly simple.

An expert witness brings two kinds of expertise to court: expertise in their professional field, and expertise in performing the role of an expert witness.

The first does not automatically provide the second.

Judgment

A High Court decision provides a useful warning for professionals who become expert witnesses in matters in which they have already been involved.

In MSA Properties Ltd v The Drapery Northampton Ltd [2026] EWHC 1420 (KB), the court had to resolve a dispute concerning deteriorated structural joists in a building in Northampton. Both parties relied on structural engineering evidence.

The point of wider interest for experts concerns the defendant’s expert, who had already been involved in the matter before the litigation.

He inspected the building in 2023 and produced a report for the defendant. That report was not prepared as expert evidence for the proceedings. By 2025, however, he was acting as the defendant’s expert witness.

That change of role became important.

The judge noted that the later expert report did not refer to the earlier inspection or the report produced at the time, despite the expert accepting in oral evidence that the earlier inspection had given him a better view of the joists.

The court was also critical of the extent to which he had engaged with the available evidence and of the opportunities available to him to inspect the property more fully.

The most significant criticism was the judge’s observation that his approach reflected:

“some conflation of his role as a contractor engaged in negotiations with the Claimant and his duties as an expert witness”

There was no suggestion that the expert had deliberately acted partially. The concern was rather that aspects of his earlier role appeared to have carried over into the way he approached his later expert evidence.

That is a useful warning for any professional who changes role during the life of a dispute.

Previous involvement does not automatically prevent someone from later acting as an expert witness. In some cases, that earlier involvement may mean they have valuable knowledge of the subject matter.

But the nature of the role changes.

A contractor, consultant or adviser may previously have been helping a client solve a problem, negotiate a position or achieve a particular commercial outcome. An expert witness has a different duty.

Under CPR 35.3, an expert’s duty is to help the court on matters within their expertise. That duty overrides any obligation to the person from whom they have received instructions or by whom they are paid.

That is not simply a formal statement to be repeated in the declaration at the end of a report. It has practical consequences for the way an expert approaches the evidence.

Transparency about previous involvement

Where an expert has already inspected the subject matter, produced reports, advised a party, taken part in negotiations or expressed views before being instructed as an expert witness, that history may be material to the opinion now being given.

The point is not that every earlier conversation or document must automatically be reproduced. It is that relevant prior involvement should not be left invisible if it bears on the expert’s present reasoning.

That is particularly important because CPR 35.10 requires an expert report to state the substance of all material instructions, whether written or oral, on the basis of which the report was written. Practice Direction 35 also requires the expert to make clear which facts are within their own knowledge and which have been provided by others.

In this case, the earlier involvement was not peripheral. The expert had inspected the joists at a time when they were more fully exposed and had produced an earlier report. He later accepted that the earlier inspection had given him a better view of the joists, yet neither the inspection nor the earlier report appeared in his expert report.

That is a useful illustration of why disclosure matters.

If earlier involvement has informed what the expert knows, assumes or concludes, the court should be able to see that route clearly. If it only emerges later, particularly in cross-examination, the omission can create an avoidable question about the completeness and transparency of the expert’s reasoning.

For an expert changing role, a sensible question at the outset is:

What have I already done, seen, said or concluded in this matter, and what of that is material to the opinion I am now giving?

If someone changes role during the life of a dispute, earlier assumptions may need to be revisited. Previous opinions should not simply be carried forward without being tested against the evidence now available. Earlier inspections, reports and involvement may themselves be relevant and should be dealt with openly where necessary.

The wider framework of the Civil Procedure Rules matters as well.

The overriding objective in CPR 1.1 is to enable the court to deal with cases justly and at proportionate cost. Expert evidence forms part of that process. It is therefore not enough for an expert merely to possess relevant expertise. The evidence has to assist the court in a way which is independent, properly reasoned and proportionate to the issues in dispute.

That is why a change in role can be important.

A professional may quite properly have spent months or years advising a client, negotiating with another party or working towards a particular practical outcome. Those activities may require judgement, persuasion and a close identification with the client’s interests.

The expert witness role is different.

Once instructed as an expert, the professional must be able to step back from that earlier involvement and approach the evidence independently. That may mean reconsidering previous views, acknowledging matters which do not support the client’s position and making clear where earlier involvement has informed the present opinion.

• The Academy’s guidance on party-appointed experts explains the overriding duty owed by an expert to the court.
• The Academy’s Code of Practice for Experts places independence, impartiality, objectivity and integrity at the heart of expert practice.
• The Academy’s Model Form of Expert Report is also relevant where an expert has had previous involvement, because the report should make clear the factual basis of the opinions expressed and the material relied upon.

The lesson from MSA Properties is not that previous involvement disqualifies an expert. It is that a professional who moves from contractor, adviser or negotiator to expert witness must make that transition consciously and transparently. The court may look closely at whether the expert has genuinely separated the new role from the old one, and whether the report gives a clear account of any earlier involvement which bears on the opinion now being expressed.

For experts, that is a useful point to consider before accepting instructions in a matter in which they have already been involved.

See the judgment here on BAILII.

Anthropic has just announced that Claude will start leaving an invisible mark in the text it generates, in an announcement that may have unintended consequences for expert witnesses.

Supported Claude models will embed machine-readable watermarks into generated text. Anthropic says those marks will travel with the words when copied and pasted into other documents and may survive subsequent editing.

The immediate purpose is provenance. As the volume of AI-generated material increases, watermarking offers a potential means of identifying content that has passed through a particular AI system.

For expert witnesses, however, that raises a rather more difficult question: if an expert report contains an AI watermark, what exactly does that prove?

Perhaps less than might first appear.

Detection is not the same as authorship

One of the most important qualifications comes from Anthropic itself.

Anthropic cautions that detecting its watermark does not establish that Claude wrote or originated the material. Rather, it indicates that the content may have been processed by Claude.

That distinction could be particularly important in expert evidence.

An expert might write an entire report themselves and subsequently ask an AI tool to correct spelling and grammar. Another might use AI to improve the structure of material they have already written. A third might provide their analysis and ask an AI system to turn it into polished prose.

Those activities are quite different from asking an AI system to examine the underlying evidence, perform the analysis and suggest the opinion the expert should express.

Yet all involve AI-generated or AI-processed text.

Consider four possible uses:

• An expert writes their report and uses AI to check spelling and grammar.
• The expert writes the substance but asks AI to improve its structure or expression.
• The expert provides their analysis and asks AI to draft passages expressing it.
• The expert asks AI to analyse the evidence and assist in formulating the opinion.

All involve AI, but they plainly do not raise the same questions about the independence of the expert’s analysis and opinion.

That makes the presence of a watermark potentially useful evidence, but not necessarily evidence of what might initially be assumed.

Provenance of the text is not provenance of the opinion

This is perhaps the central issue for expert witnesses. A watermark may provide evidence about the provenance of the words. It might indicate that Claude, or another AI system using similar technology, has been involved in generating or processing them:

• It cannot, by itself, establish the provenance of the opinion.
• It does not tell us who examined the evidence.
• It does not tell us who identified the relevant issues.
• It does not tell us whether the expert carried out the underlying calculations or analysis.
• And, most importantly, it does not tell us whether the conclusion was independently reached by the expert or suggested by an AI system.

The same paragraph could conceivably carry an AI watermark whether Claude corrected three grammatical errors in it or generated the paragraph from scratch.

For expert evidence, those are very different propositions.

The absence of a watermark proves little too

There is also a problem in the opposite direction.

No watermark does not necessarily mean no AI.

Anthropic acknowledges that detection can become more difficult where generated material has subsequently been heavily edited, paraphrased, translated or combined with other text. A passage may also simply be too short to provide sufficient information for reliable detection.

This creates an important limitation if watermark detection ever begins to be used when examining expert reports.

A positive result may indicate AI involvement without establishing the extent or nature of that involvement whilst a negative result cannot necessarily establish that there was no AI involvement at all.

Watermark detection should therefore not be treated as a binary test of whether an expert “used AI”.

A new question in cross-examination?

The development also raises an interesting practical possibility.

“Your report contains a watermark indicating that it was processed by Claude. How did you use it?”

The answer might be entirely unremarkable.

The expert might explain that they wrote the report themselves and used Claude to check its grammar. They might explain that they used it to condense a lengthy passage they had already written. Alternatively, the answer might reveal that AI played a much greater role in analysing material or formulating the opinions contained in the report.

The takeaway is that whilst the watermark itself cannot distinguish between those situations, it may, however, provide the starting point for questions which can.

That means the increasingly important question may not simply be:

“Did you use AI?”

It may instead be:

“Precisely how did you use it?”

Watermarking is becoming a real technology

Anthropic is not alone in pursuing this approach.

Google has already developed SynthID Text, a watermarking technology for AI-generated text. Rather than inserting a visible label or conventional metadata, SynthID subtly affects the model’s choice of tokens as text is generated. Across a sufficiently long passage, those choices create a statistical signature which can subsequently be detected.

Anthropic has not publicly confirmed that Claude’s watermark uses the same technical method, so the two systems should not be assumed to work identically.

The significance is broader. Text watermarking is moving beyond academic research and into mainstream generative AI products.

If the technology becomes widespread, lawyers, courts and experts may increasingly encounter claims that a document has been identified as having passed through an AI system.

Understanding what such a finding does, and does not, establish will therefore become important.

The expert remains responsible

The Academy’s guidance on the use of Artificial Intelligence by expert witnesses already addresses the more fundamental issue.

AI can be a useful tool, but it cannot substitute for the expert’s own expertise, analysis and opinion. Experts remain responsible for the evidence they give. Where AI has played a significant role, keeping appropriate records of how it was used may also become increasingly important, particularly if the expert is subsequently required to explain that use in court.

Watermarking adds another dimension to that responsibility.

It may make some uses of AI more readily identifiable. But identifying that an AI system touched the words is not the same as establishing who produced the intellectual work behind them.

For expert evidence, that distinction is crucial.

Provenance of the text is not necessarily provenance of the opinion.

As AI provenance technology develops, the ability of an expert to explain what the AI did, what the expert did, and where the opinion actually came from may become increasingly important.

In Trinsic Collagen Ltd v Procoll Ltd [2026] EWHC 1793 (Ch), the court criticised the way additional information had been provided to the claimant’s experts and then relied on without being clearly identified in the reports, or in one instance the joint statement.

The issue was not that the experts had spoken to the client. It was that the factual basis of their conclusions was not always transparent.

For an expert report to assist the court properly, the reader should be able to see the route from the facts and assumptions, through the analysis, to the opinion reached. That means being clear about material obtained through discussions, site visits, correspondence or subsequent instructions, especially where it influences the expert’s reasoning.

The Academy’s own guidance reflects the same principle. An expert report should contain a concise statement of the facts and assumptions used, followed by the expert’s analysis and opinion, with fact and opinion clearly distinguished.

A useful question before signing a report is this: could another expert, or the judge, identify every material fact and assumption on which my conclusion depends? If not, the report may need another look.

The Academy’s Model Form of Expert Report also reflects this approach.

Not later. In the report.

If an expert opinion depends on incomplete data, assumptions or qualifications, those limitations belong in the written report.

1. Not in cross-examination.
2. Not in re-examination.
3. Not only after the tribunal asks the right question.

In Mew v General Dental Council [2026] EWHC 1116 (Admin), the High Court considered expert evidence given during professional disciplinary proceedings.

One expert had deliberately avoided setting out limitations in the available data, believing that they could instead be explained when he gave oral evidence.

Charles Bagot KC, sitting as a Deputy High Court Judge, said that this revealed a “fundamental misunderstanding” of the obligations of a CPR-compliant expert.

The problem was not that the evidence required qualification. Many expert opinions do.

The problem was that the qualification had been withheld from the report.

A court or tribunal must be able to understand:

• what the expert can conclude;
• what the expert cannot conclude;
• where the evidence is uncertain; and
• how those limitations affect the strength of the opinion.

A carefully expressed qualification does not weaken expert evidence. It allows the decision-maker to assess it properly – and will often make the opinion more credible.

The expert report should contain the opinion the expert can genuinely support, not an unqualified version that appears more persuasive on first reading.

Read the judgment:
https://www.bailii.org/ew/cases/EWHC/Admin/2026/1116.html

Expert witnesses play a key role in the justice system and it has always been the case that expert reports and other documents referred to in court were in the public domain. But getting hold of them has not been straightforward and concerns were raised in Lady Hale’s judgment in Cape Intermediate Holdings Ltd v Dring [2019] UKSC 38 a  case relating to asbestos exposure. This set out something of a mish-mash of practices and rules and the issue, together with the increasing reliance on electronic court bundles gave rise to the work of  Transparency and Open Justice Board.

The resulting Access to Public Domain Documents Pilot, postponed from earlier this year, will now go live on 1^st January 2026, initially for year. Details of the pilot are set out in Practice Direction 51ZH but the key points that experts should take note of are as follows;

• Pilot covers Commercial Court and London Circuit Commercial Court of the King’s Bench Division and  the Financial List (Commercial Court and Chancery Division) only.
• It affects any documents that have already been filed with the court and which are then relied on in court from January 2026, not just new documents.
• Affected documents include expert reports and annexes and appendices to expert reports;
• Expert reports these must be filed within 14 days of being relied on (submissions, skeletons within 2 days).

Filing of reports will be the norm, exceptions require a Filing Modification Order or FMO to be applied for.

In summary while expert reports were always theoretically available to non-parties access should become a great deal easier, see the Academy website for more detailed guidance.

Only time will tell whether the press or public will have an appetite for reading sometimes complex and lengthy reports on forensic accounting, construction or IT but experts should make sure, as always, that their opinions are properly supported, reasoned and consistent with their previous opinions – remembering that any report that has been filed will potentially become available through the pilot if used in hearing!

The Academy responded to the consultation for the pilot and continues to engage with the judiciary and legal profession on this and other issues to ensure that expert views are taken into account and that members have access to the best possible advice and guidance.

Our Judicial Committee publishes the authoritative guidance for experts so you can be sure that you are adopting best practice. This includes for example the Model Form of Expert’s Report and Guidance Notes on Meetings of Experts, Contingency Fees & Remote Evidence.

We also provide practical advice and guidance on topics such as GDPR, Joint Statements, privilege and so on which is available at no charge to members in the Academy’s Knowledge Hub.

Members can also call our Technical Helpline for help and advice on ethical and practical issues from dealing with instructions to getting paid.