Skip to main content

Accessibility controls

Main content area

Disclosure Manual: Chapter 36 - Expert Witnesses - Prosecution Disclosure Obligations

Refreshed: 21 October 2021|Legal Guidance


The test for disclosure of unused material is the same in relation to material generated by an expert as for all other types. If unused material relating to an expert witness is relevant, the police must reveal it to the prosecutor and, if the material meets the disclosure test, it must be disclosed to the defence, or a PII application made.

There is no definitive legal definition of an expert. It is a matter for the court to rule upon in each case. However, for the purposes of this guidance, an expert is defined as a person whose evidence is intended to be tendered before a court and who has relevant skill or knowledge achieved through research, experience or professional application within a specific field sufficient to entitle them to give evidence of their opinion and upon which the court may require independent, impartial assistance.

The difference between an expert and other witnesses is that experts are the only witnesses allowed to give opinion evidence. For that reason, an expert witness's competence in their field of expertise may be in issue, as well as their credibility. If an expert's credibility and/or competence is the subject of concern, that information should be considered for disclosure.

Expert witnesses, and parties calling them, are additionally subject to the rules for experts set out in CrimPR 19 to serve with an expert’s report anything of which the expert (or the party instructing them) is aware which might reasonably be thought capable of undermining the reliability of the expert’s opinion, or detracting from the credibility or impartiality of the expert. Further details are available in the CPS guidance on expert evidence.

Guidance for experts

The obligations which apply to an expert are to ensure that the prosecution team can comply fully with the requirements of disclosure. These obligations take precedence over any internal codes of practice or other standards set by any professional organisations to which the expert may belong and can be summarised as the key actions of record, retain, and reveal.

An expert not employed by the police is a third party and is not bound by the obligations set out in the CPIA as amended. The CPS seeks to impose these obligations as part of the contractual relationship with the expert.

The obligations are set out in CPS Guidance for Experts on Disclosure, Unused Material and Case Management (the Guidance for Experts).

When an expert is instructed in an investigation, it must be ensured that the expert understands the obligations placed upon them by this status. The expert witness has an overriding duty to assist the court and, in this respect, the expert's duty is to the court, and not to the investigator or prosecutor. This will include obligations relating to disclosure.

The Guidance for Experts also contains a sample of the index of unused material that an expert will be asked to complete, describing all the unused material in their possession. The expert will not be expected to distinguish between sensitive and non-sensitive material. It is the responsibility of the disclosure officer, in conjunction with the expert, to identify any sensitive material.

The disclosure officer should include the index completed by the expert, on the MG6C/D schedule.

Competence and credibility

Investigators have a duty to pursue all reasonable lines of enquiry. This would include satisfying themselves that any witness to be called as an expert is both credible and competent. Where questions are raised as to the competence or credibility of an expert witness, members of the prosecution team should refer to Chapter 37 of this manual.

The expert's self-certificate

The expert instructed should submit to the investigating officer or disclosure officer, a completed self-certificate revealing whether or not there is information which may be capable of adversely affecting his or her competence and/or credibility as an expert. This should be submitted to the investigating or disclosure officer as soon as the expert is instructed.

Expert witnesses will be asked to complete a certificate on every occasion that the expert is asked to provide expert evidence in the form of a full statement or report. Where an expert refuse to complete a certificate, consideration should be given by the prosecution team to the use of that expert in future cases.

The prosecutor should ensure that the details contained in the certificate are sufficient to enable the disclosure officer to make an informed decision about the relevance and disclosability of the information to the proceedings in question. If it is insufficient then the prosecutor should ask the disclosure officer to make further enquiries.

Where the expert has indicated that there is an issue in answer to any of the questions on the certificate, the prosecutor is responsible for considering whether the information satisfies the disclosure test.

The certificate should be referred to on the MG6C. The certificate should be described on the MG6C as: "completed certificate by [name of expert]."

Matters that relate to the credibility and/or competence of expert witnesses, if disclosable, may be deployable in court in order to discredit their evidence.

Bad character

Prosecutors should have regard to the provisions in part 11 of the Criminal Justice Act 2003. Expert witnesses are open to questions about their bad character in the same way as any other witness. Prosecutors should refer to the CPS legal guidance on Bad Character Evidence for guidance on whether this material can be used in court.

Unresolved complaints and disciplinary proceedings

Complaints about expert witnesses, or disciplinary matters that are under investigation and have not yet been concluded, should be revealed by the disclosure officer to the prosecutor. The information that such unresolved allegations reveal may be relevant and the prosecutor should consider whether it satisfies the disclosure test.


The expert must include in their statement a declaration that they have understood and complied with their disclosure obligations. It is suggested that this declaration should appear at the beginning of the expert's statement in the section before they cite their qualifications. The declaration can be found in the Guidance for Experts.

Failure by an expert to complete the declaration may cause the prosecutor to seek another expert.

PNC checks

A PNC check should be conducted in relation to every expert witness on whose evidence the prosecution relies. Previous convictions or cautions (save for minor road traffic offences) may fall to be disclosed as part of initial disclosure. Please refer to Chapter 12 of this manual for further guidance.

Adverse judicial findings

An adverse judicial finding is a finding by a civil or criminal court, expressly or by inevitable inference, that an expert witness has knowingly misled the court, whether on oath or otherwise. With regard to experts, this definition may be extended to include unintentional misleading of the court, i.e. incompetence.

It is the duty of any advocate representing the CPS to record an adverse judicial finding in full. A transcript should be requested wherever available. Any adverse judicial finding against an expert witness should be reported by the advocate to the CPS.

There is no mechanism for the court to rescind an adverse judicial finding. However, if subsequent information comes to light that casts doubt on the finding, this should be reflected in the certificate and this will be a factor to be taken into account by the prosecutor when deciding whether, applying the disclosure test, to disclose the information.

When information is received that casts doubt over the reliability of an expert witness and/or the expert's technical area of expertise, consideration should be given to whether further disclosure is needed in current and past cases involving the expert. Prosecutors should refer to Chapter 37 of this manual for detailed guidance.


The prosecutor must first consider whether the information is relevant i.e. whether it has any bearing on the case. Only if the information is considered relevant will it need to be considered for disclosure to the defence.

If the prosecutor forms the view that the information would not satisfy the disclosure test, taking account of the nature of the expert's evidence and what is known of the defence or likely defence, details of the information submitted in the certificate should not be disclosed, but must be kept under review and reconsidered in the light of any defence statement. If a doubt remains about whether disclosure is required, the prosecutor should disclose. The decision to disclose or withhold information must be made or approved by a unit head or equivalent.

Prosecutors should cross refer to other relevant chapters in the Disclosure Manual when necessary. In particular, prosecutors should refer to Chapter 23. Prosecutors should also refer to the CPS guidance on expert evidence and note the additional rules for experts set out in CrimPR 19.

Record of decision

The decision and details of any consultation between CPS prosecutors, the prosecution advocate and/or the police must be fully recorded on the Disclosure Record Sheet.

Notification of decision to the expert

On the date of trial, or earlier if possible, the prosecutor or caseworker in court will inform the expert whether the information has been disclosed or withheld.

Change of circumstances

It is essential that the prosecutor can, so far as possible, be confident that the information provided by the expert on the certificate is up to date. This is particularly the case where the hearing of a trial or appeal is imminent and some time has passed since the expert submitted the certificate.

Scroll to top