How often do you see an expert witness themself become part of the reason a criminal conviction is overturned?

That is, in effect, what happened in The King v ZX [2025] NICA 76, a decision of the Court of Appeal in Northern Ireland concerning convictions for murder and attempted murder.

Psychiatric evidence was central to the trial. The defendant’s mental state was directly relevant to the issues the jury had to decide, and both the prosecution and defence relied on psychiatric experts.

What makes the case particularly striking is not simply that the Court of Appeal disagreed with an expert’s opinion. Courts do that all the time. The problem went much deeper. The court concluded that the prosecution psychiatrist’s evidence had evolved in such a way that it could no longer be regarded as the independent product of an expert uninfluenced by the demands of the litigation.

The expert had, in the court’s words, effectively assumed an impermissible role in promoting the prosecution case.

That is an unusually serious finding against any expert witness.

An opinion which developed during the trial

Before trial, the prosecution psychiatrist had assessed the defendant as suffering from depression, although not to the degree suggested by the defence expert. During his oral evidence, however, he introduced a much more significant proposition: that the most likely explanation for what had happened was “spousal revenge”.

That theory had not appeared in his reports or his contemporaneous notes. It emerged in evidence after the expert had been provided with research material addressing that subject by prosecution counsel.

There were other examples of important matters appearing for the first time at trial. The expert placed weight on the condition of the defendant’s home as an indicator against severe depression, although that had not featured in his reports or notes. He also relied on the severity of her self-inflicted injuries in a way which had not previously been identified.

Experts are, of course, allowed to change their opinions. New information may emerge or further consideration may lead to a different conclusion. But a material change of view should be identified and communicated. A party should not discover an important new expert opinion for the first time when the expert is giving evidence.

That principle was central to the Court of Appeal’s reasoning.

A quotation which changed meaning

Perhaps the most uncomfortable factual detail concerned a quotation attributed to the defendant.

The expert’s report recorded her as saying:

“he destroyed my life, so I destroyed his”

When the expert’s original handwritten notes were eventually disclosed during the trial, however, they recorded only:

“he destroyed my life”

There was also a discrepancy about when the statement was said to have been made.

The difference is obvious. Adding the second half of the sentence changes a statement about what had happened to the defendant into something which appears retaliatory. That was particularly significant given the later emergence of the “spousal revenge” theory.

The court recorded that, when challenged in cross-examination, the expert at one stage agreed that he had lied about what had been said, although he later resiled from that position.

Whatever the explanation, it illustrates why an expert’s original notes can matter just as much as the final report.

Material which had not been considered

The problems were not confined to the development of new theories.

The expert accepted that he had not considered police body-worn footage, had not asked to see it and had not regarded it as part of his assessment. He had also failed accurately to record when, and to what extent, the defendant had received antidepressant medication. When these omissions were put to him, he maintained that they did not alter his opinion.

That poses a familiar problem for experts. A conclusion may ultimately remain the same after additional material is considered, but that is different from never having considered the material in the first place.

An expert needs to be able to explain not only the conclusion reached but the evidential basis on which it was reached, including material which points in the other direction.

The relationship between the expert and the legal team

The Court of Appeal was also highly critical of the way in which the prosecution expert had been handled.

Before giving evidence, the expert had a telephone conversation with prosecution counsel lasting between approximately 45 minutes and an hour. Neither participant made a note or record of the conversation. A further meeting took place on the day the expert began giving evidence.

The concern was not simply that discussions had taken place. Conferences between lawyers and experts are normal and often essential. The difficulty was what happened afterwards.

The court noted that matters raised during the consultation subsequently became prominent in the expert’s evidence and increasingly supportive of the developing prosecution case. It said this gave rise to an appearance of coaching or of the expert having been consciously or subconsciously influenced.

Importantly, the court did not make a finding of deliberate coaching, but it did find that the prosecution’s conduct had fallen below recognised standards of good practice.

There is a useful distinction here. Lawyers are entitled to test an expert’s opinion, challenge assumptions, ask questions and introduce new material. What they cannot properly do is shape the expert’s evidence into the case they want to present.

The expert has the corresponding responsibility to resist that process.

No written letter of instruction

Remarkably, the prosecution psychiatrist had no written letter of instruction.

The Public Prosecution Service explained that he had been given a brief verbal explanation of the case, but nothing had been put in writing.

The Court of Appeal’s response was blunt. It described that as “patently not good enough”, said that the practice lacked transparency and stated that in future experts in criminal cases should have a written letter of instruction.

For experts used to civil litigation, that may sound like an obvious requirement. But the reason for it goes beyond administration.

Written instructions establish what the expert was originally asked to consider. They help define the issues, provide a record of the material supplied and make it possible later to see how and why an opinion developed.

Without that starting point, it becomes much harder to distinguish independent analysis from a case theory which may have developed through conversations with the instructing team.

The court’s conclusion on independence

The Court of Appeal returned to well-established principles governing expert witnesses.

Expert evidence should be the independent product of the expert. An expert should provide objective and unbiased assistance to the court, should identify the facts and assumptions supporting an opinion, should not omit material facts which detract from it and should communicate any material change of view promptly.

Applying those principles, the court concluded that the prosecution psychiatrist had departed significantly from his role as an independent expert and had effectively assumed an impermissible role promoting the prosecution case.

The consequences were serious.

The court said that without the perseverance of defence counsel in obtaining the expert’s notes and examining his dealings with prosecution counsel, there was a real possibility that the deficiencies and partiality in the evidence would never have been exposed. It said that this could have led to a miscarriage of justice.

The ground of appeal relating to the expert evidence succeeded.

The responsibility did not stop with the expert

It would nevertheless be wrong to treat ZX simply as a case about an expert who lost his independence.

The Court of Appeal also found serious problems in the conduct of the prosecution and in the way the trial judge dealt with the difficulties once they had emerged.

The prosecution’s handling of the expert was criticised, including the informal nature of the instructions, delayed disclosure and the interaction between counsel and the expert. The ground of appeal relating to prosecutorial misconduct also succeeded.

The court also found that the trial judge had failed to give sufficient consideration to applications to discharge the jury. It concluded that the jury could easily have been misled by the expert evidence and that directions could not have cured the resulting prejudice.

Both convictions were quashed.

The court’s overall conclusion was that the serious errors began with the prosecution’s misconduct and were compounded by the expert witness and the judge.

Could a new expert start again?

There is one final aspect of the case which is particularly interesting for anyone involved in expert evidence.

Once the convictions had been quashed, the court had to consider whether a retrial could take place. One difficulty was that the “spousal revenge” theory had by then become embedded in the case papers.

The Court of Appeal was concerned that any new expert might themselves be influenced by that history simply by reading the existing material. It described the risk of the replacement expert becoming “contaminated”.

When a retrial was eventually ordered, the court directed that the new expert’s letter of instruction should, so far as possible, be prepared on a “blank page basis”.

That is perhaps one of the most thought-provoking parts of the judgment.

Expert independence is not only about resisting pressure from the party instructing you. It also means being alert to the way in which the material provided to you can frame the problem before you have even begun your own analysis.

Independence can be lost gradually

Most experts know that their duty is to the court rather than to the person paying their fees. It is one of the first principles of expert witness work.

What ZX demonstrates rather starkly is how independence can be eroded without there necessarily being one obvious moment when the line is crossed.

A discussion takes place. A new theory is raised. A piece of research is introduced. Something which seemed insignificant in the report becomes central in oral evidence. Material which does not fit the emerging theory receives less attention. Gradually, the expert stops simply assisting the court and begins to move with the case being advanced by one side.

That is why the disciplines around expert evidence matter: clear written instructions, complete records, contemporaneous notes, proper disclosure, transparency about changes of opinion and a willingness to consider evidence which does not support the preferred conclusion.

None of those things is glamorous. They are, however, part of what keeps an expert independent.

In ZX, their absence contributed to something very unusual indeed: the expert’s own conduct became part of the reason two criminal convictions could not safely stand.

Case: The King v ZX [2025] NICA 76, Court of Appeal in Northern Ireland.

Read the judgment

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.

A new Right to Work regime takes effect from 1st October 2026 and may affect experts, their use of associates and, potentially, the terms agreed with instructing solicitors and expert agencies. With civil penalties of up to £60,000 per illegal worker, this is an area where mistakes can prove costly.

The UK government has required employers to carry out checks on employees’ right to work since the Immigration, Asylum and Nationality Act 2006. Section 48 of the Border Security, Asylum and Immigration Act 2025 extends that regime so that, in some circumstances, civil penalty liability can reach beyond the employer who has the direct contractual relationship with the worker.

It is this extension that is particularly relevant to experts, their associates and those who instruct them. The Home Office has issued draft guidance setting out how the new regime is expected to operate.

Associates engaged directly

If you or your expert firm engage an individual associate or subcontractor to help deliver work, you may need to carry out a prescribed Right to Work check before that person starts work.

The position depends on the nature of the arrangement. The new regime is not intended to mean that every self-employed expert or sole practitioner automatically has to be checked. However, where an individual is engaged to help fulfil work which your firm has itself contracted to provide, the arrangement may fall within the extended regime.

The guidance explains the different ways in which a Right to Work check can be carried out. Depending on the circumstances, this may involve a manual document check, a Home Office online check or the use of an approved digital verification service provider. Records may also need to be retained and, where the individual has time-limited permission to work, further checks may subsequently be required.

Associates engaged through another firm

If you or your firm engage associates through their firm or another organisation, there is a separate issue: extended liability.

Under the new regime, liability may in some circumstances extend further up a contractual chain. A business which does not itself employ the individual may nevertheless be exposed if illegal working is later identified.

The guidance provides for a statutory excuse where the relevant requirements are met. These include having appropriate contractual terms in place which require prescribed Right to Work checks to be carried out, control further subcontracting, allow appropriate compliance checks and provide for action to be taken where illegal working is identified. The relevant contractual requirements are set out at page 49 of the current draft Home Office guidance.

The protection is not simply a matter of inserting standard wording into a contract. The arrangements also need to operate properly in practice.

What does this mean for instructing solicitors and expert agencies?

Whilst the immediate focus for experts and expert firms is likely to be the checking of individuals and the review of associate or subcontract terms, the changes may also have consequences for agreements with instructing solicitors and expert agencies.

Those further up the contractual chain may themselves wish to establish a statutory excuse against extended liability and may therefore seek additional contractual assurances from experts about Right to Work compliance, subcontracting and the use of associates.

It would not therefore be surprising to see new Right to Work provisions appearing in expert instructions and agency terms after 1 October.

What should expert firms do now?

Expert witnesses and expert firms that use associates or subcontractors should review their arrangements before the new regime takes effect.

In particular, they should consider:

• whether any individuals they engage may fall within the extended Right to Work regime;
• how Right to Work checks will be carried out where required;
• whether existing associate or subcontract terms need updating;
• whether their contracts permit substitution or further subcontracting; and
• whether clients or agencies are likely to require additional contractual assurances.

The timing is slightly more nuanced than saying that every existing agreement has to be rewritten by 1st October. The new provisions apply to relevant engagements commencing on or after 1st October 2026, and to relevant contractual arrangements entered into on or after that date.

Nevertheless, firms which regularly use associates or subcontractors would be well advised to review their procedures and standard terms now, rather than waiting until a new instruction or engagement arises.

The Home Office guidance is currently in draft and should be checked alongside the legislation before any changes are made to contractual arrangements.

Further information

The Home Office’s current guidance is available in its Draft employer’s guide to right to work checks: 11 September 2026.

This article is intended as a general overview of the changes and does not constitute legal advice.

Being instructed as an expert witness can be an interesting and fulfilling aspect of professional life, but it’s not without its challenges. One of the most common (and under-discussed) issues experts face is how to manage their instructing solicitors.

It’s easy to assume that once you’re engaged, expectations are fixed and your role is simply to deliver. But in reality, managing your instructing solicitor is a key part of ensuring that what you deliver is useful, compliant, and capable of standing up in court.

Academy Council Member Helen Gregory of Opus Forensic Accounting has helpfully provided her five top tips for how expert witnesses can work most effectively with their instructing solicitors.

1. Be realistic about deadlines — and say no if needed

There’s a temptation to take on every instruction that comes your way, particularly if you’re self-employed and don’t know where the next one is coming from. But accepting unrealistic deadlines can cause problems for all involved. Rushed reports are more likely to contain errors, lack clarity, or fail to meet procedural requirements. And in the worst-case scenario, they can be thrown out by the court entirely.

It’s better to decline a job than to submit something half-baked. Be clear at the outset about what’s feasible and remember to factor in time not just for writing the report, but for reviewing documents, requesting further information, meetings with the solicitor and client, responding to queries, and navigating any surprises.

2. Build in contingency: Something will crop up

In theory, you receive a neat, complete bundle of documents and a clear brief. In practice, things are rarely that tidy. Documents arrive late or in multiple batches, some are missing or out of order, and the questions you need to address will take time to answer.

You need to allow for contingency: time to request clarifications, review additional information, and think critically about what you’re being asked to do. The more flexibility you build in, the more likely you are to produce a report that is robust, complete, and helpful to the court.

3. Understand the limits of your visibility

One key challenge is that, as an expert, you often operate in isolation from the wider case. You may see only a snapshot of the full picture. It’s important to remain focused on your instructed area and resist the temptation to speculate beyond it.

At the same time, remember that lack of context doesn’t remove your duty of care. If documents appear incomplete, if your opinion feels influenced, or if something doesn’t sit right, you must raise it. You are reporting to the court, not acting as an advocate for either party.

4. Deliver bad news where it’s warranted

It’s not always easy to say your client doesn’t have a case. But as an expert witness, you have a duty to give your honest, professional opinion, even if that opinion isn’t what the instructing solicitor wants to hear.

Be aware that some may (consciously or otherwise) attempt to steer your opinion. Stay alert to missing evidence, biased questions, or subtle attempts to frame a narrative. Your integrity and credibility depend on your ability to remain independent and evidence-based.

5. Know the rules and review your report thoroughly

Expert reports must comply with the relevant Civil, Criminal or Family Procedure Rules. That includes formatting, content, and declarations. A technical report that omits the correct statement of truth or includes the wrong version risks being ruled inadmissible.

It’s also essential to re-read what you’ve written. That may sound obvious, but when deadlines loom and multiple drafts are flying around, it’s easy to miss a critical detail. The best option is to ask someone else unconnected to the case to read and cast it, but that may not always be possible so do make time to review your own work with a clear head.  You may be cross-examined on your report, so make sure you’re comfortable defending it.

Final thoughts

Whatever your discipline, acting as an expert means more than just knowing your subject. It requires planning, time management, self-awareness, and the confidence to push back when needed.

Clear communication with instructing solicitors, realistic boundaries, and careful preparation are vital. If in doubt, The Academy of Experts offers training, guidance and support to help navigate the more complex aspects of expert witness work.

Helen is a Partner at Opus Forensic Accounting, part of the Opus Business Advisory Group, and brings over 25 years’ experience in civil and criminal investigations, commercial disputes, and matrimonial matters. A trusted expert witness and active figure in the sector, including roles with The Academy of Experts and the South West Fraud Forum, she recently joined Opus, a national advisory firm with 14 offices, 35 Partners, and specialist divisions spanning restructuring, strategic advisory, forensic accounting, and equity.

 

Two recent articles in the Guardian cited the appeal court’s over-turning of a doctor’s conviction and an assortment of criticisms of experts made over recent years in highly critical articles, calling for compulsory training and registration of experts. Our Chair, William Hooper responded on our behalf as follows;

Recent articles have discussed concerns about expert evidence in the Courts and argued for greater regulation.

An expert witness is someone with specialist knowledge which the Court considers useful to itself in resolving a case.  The expert’s task is to explain the significance of technical material to the Court so that the judge (in a civil case) or jury (in a criminal case) can understand it and include it as part of their considerations. The decision-makers weight evidence, including that from experts, as they see fit.

The expert witness does not decide the case in either the civil or criminal courts.

Technical evidence can be complex.  It often happens that expert witnesses appointed by the two sides agree on some points of interpretation and disagree on others.  Expert witnesses may be subjected to robust cross-examination to detect error, incompetence or the hired gun.

Your writer calls for greater regulation but appropriate regulation has been introduced.  Instructing solicitors are required, when instructing experts, to inform them of their duties under the Civil (or Criminal or Family) Procedure Rules and the Practice Directions.  These cover the minimum legal obligations for expert witnesses.

The Academy of Experts, since 1987 a professional and accrediting body for expert witnesses, agrees that training is appropriate to ensure that those appointed fully understand their obligations and how those apply in a variety of potentially difficult situations.  Such training has been available for many years from The Academy of Experts and other sources.

Rather than the imposition of additional regulation, which may result in the exclusion of experts with the best experience and expertise to assist the Court, it is to be hoped that the CPS and instructing lawyers will carry out appropriate due diligence and choose to instruct properly qualified and trained expert witnesses.

William Hooper

Chair, The Academy of Experts

3 Gray’s Inn Square, London WC1R 5AH