This guide is maintained as a current resource for September 2026 and covers only the laws of England and Wales. Information is for general guidance, not legal advice. Consult a qualified solicitor for advice specific to your situation.
Learn how expert witnesses are identified, instructed and presented in criminal trials in England and Wales. This guide explains roles, legal rules, reporting, disclosure responsibilities and trial procedures for expert evidence in criminal courts.
Expert witnesses play a crucial role in the criminal justice system in England and Wales. Where a case involves specialised knowledge beyond what a judge, magistrate or jury can reasonably be expected to understand, expert evidence may be necessary to explain complex scientific, medical, technical or professional issues. This guide explains how expert witnesses are identified, instructed, disclosed and presented in criminal trials, using current legal rules and authoritative practice guidance. It is aimed at readers with varying levels of legal knowledge, from members of the public to solicitors and students.
Why Expert Witnesses Matter
In criminal trials, the prosecution and defence must prove or challenge facts with evidence that the court can properly evaluate. Evidence of expert opinion assists the fact‑finder (judge or jury) on matters that require specialised expertise - for example, forensic analysis, medical causation, accident reconstruction or digital data interpretation. Expert witnesses do not provide factual eyewitness accounts; instead, they give informed opinions based on detailed training and experience. Their role is to help the court understand complex issues that fall outside ordinary knowledge.
What Is an Expert Witness?
An expert witness is a professional or specialist with recognised qualifications, training or experience in a particular field. In criminal proceedings, their evidence is admissible only where the subject matter requires specialised opinion beyond what an ordinary person could reliably assess. This is set out in the Criminal Practice Directions 2023 and the Criminal Procedure Rules 2020 (Part 19).
Experts have a duty to the court, meaning their overriding responsibility is to assist the judge or jury impartially and objectively, even if they are instructed by one party. They must disclose anything that might reasonably be thought to undermine their opinion or credibility.
Deciding to Instruct an Expert
When Expert Evidence Is Needed
Expert evidence is appropriate when:
- The subject matter is outside common experience and requires specialised skill or knowledge.
- The court needs help interpreting scientific data, technical analysis or professional standards.
- The case cannot be fairly determined without an expert opinion.
Before instructing an expert, the prosecution or defence should consider whether expert evidence is necessary at all and, if so, precisely what issues the expert needs to address.
Selecting a Suitable Expert
Neither the Crown Prosecution Service (CPS) nor the defence maintains a formal “register” of expert witnesses, and there is no statutory accreditation regime for all experts in criminal cases. However, prosecutors and defence lawyers will assess potential experts based on recognised qualifications, relevant experience and ability to produce clear, reliable reports. Professional bodies, credible databases and recognised forensic standards (such as those set by the Forensic Science Regulator and UKAS accreditation) are typical sources for identifying expert witnesses.
When selecting an expert, legal teams consider factors such as:
- Does the expert have the right expertise for the issue at hand?
- Is the expert capable of giving clear, unbiased opinion evidence?
- Has the expert complied with recognised professional standards and guidelines?
Instructing the Expert
Once identified, an expert is formally instructed through a written letter (often called a letter of instruction) that sets out:
- The background and context of the case.
- The specific questions or issues on which an opinion is sought.
- The factual materials the expert should consider.
- Timelines and expectations for delivery of a written report.
The letter of instruction should make clear the scope of the expert's function and emphasise their duty to the court rather than to the instructing party.
Experts often complete a self‑certificate confirming they understand their duty to the court and have no undisclosed conflicts of interest. This self‑certification is provided to investigators or disclosure officers.
The Expert's Report and Disclosure
Producing a Report
After instruction, the expert will prepare a written report outlining:
- Their qualifications and experience.
- The evidence and materials they reviewed.
- Their analysis and conclusions.
- The basis for their opinion, supported by reference to recognised methods and standards.
The report focuses on opinion evidence relevant to the legal issues, presented clearly so that a judge or jury can understand the reasoning.
Disclosure Obligations
Under the Criminal Procedure Rules 2020 (CrimPR 19.3), expert reports must be served on the court and the opposing party as soon as is practicable. This includes providing anything that might reasonably undermine the reliability or impartiality of the expert's opinion. The party relying on the expert evidence must also offer the opposing side reasonable access to any tests, experiments or underlying material relied upon in the expert's conclusions.
Failure to comply with these disclosure obligations can lead the court to refuse to admit the expert evidence unless permission is granted.
Joint Experts and Case Management Conferences
Where both prosecution and defence intend to call expert witnesses on the same issue, the court may order experts to engage in a joint expert process. Under CrimPR 19.6–19.8, experts can be directed to prepare a joint report identifying areas of agreement and disagreement, which helps narrow the issues for trial and assists the jury in focusing on the key points.
Formal case management conferences with the judge may also be used to plan expert evidence, prevent unnecessary duplication, and limit the scope of contested issues.
Expert Witness Evidence at Trial
At trial, expert witnesses may be called to give oral evidence in addition to their written report. Their testimony helps clarify technical matters for the jury or judge and allows them to be cross‑examined by opposing legal counsel. The expert must remain independent, explaining both supporting and conflicting aspects of their opinion.
Occasionally, statute allows a report to be admitted without calling the expert to give oral evidence, but this requires leave of the court, which will consider fairness to the defence and the potential to challenge the expert's conclusions.
Practical Considerations and Challenges
Costs and Logistics
Expert evidence can be costly and is often paid for by the party instructing the expert. In criminal cases, the CPS typically covers the cost of work related to trial presentation, while investigative costs at the police stage are met by the police. Agreements on fees should be reached before work begins to avoid disputes.
Credibility and Reliability
An expert's credibility may be challenged through cross‑examination or by presenting competing expert evidence. Judges increasingly emphasise the need for expertise to be based on robust methodology, impartial analysis and transparent reasoning. Recent commentary in the legal community highlights concerns about the quality and oversight of expert evidence, reinforcing the importance of rigorous selection and preparation of expert witnesses.
Key Takeaways
The process for calling expert witnesses in criminal trials in England and Wales involves careful consideration of whether expert evidence is necessary, selecting suitably qualified and impartial experts, clearly instructing them, and complying with strict disclosure obligations under the Criminal Procedure Rules 2020. Expert reports must be served on the court and opposing parties, and experts may be called to give oral evidence at trial. Effective management of expert evidence, including joint expert procedures, helps ensure that complex issues are presented clearly and fairly, assisting the court in reaching just outcomes.
