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Chapter 36: Expert Witnesses - Prosecution Disclosure Obligations


36.1 The test for disclosure of unused material is the same in relation to material generated by an expert as for all other types of unused material. 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.

36.2 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".

36.3 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 witnesses' 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.

Guidance Booklet for Experts

36.4 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. These obligations can be summarised in the key actions of record, retain and reveal.

36.5 An expert not employed by the police is a third party and is not bound by the obligations set out in the Criminal Procedure and Investigations Act 1996 as amended. The CPS seeks to impose these obligations as part of the contractual relationship with the expert.

36.6 The obligations are set out in a booklet known as Disclosure: Experts' evidence and unused material (the Guidance Booklet). It can be found at Annex K n this Manual.

36.7 When a member of the Prosecution Team instructs an expert 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 Prosecution Team. This will include obligations relating to disclosure.

36.8 In addition to an explanation of the disclosure regime, the Guidance Booklet contains a flowchart which illustrates the process of revelation.

36.9 The Guidance Booklet also contains a sample of the index (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.

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

Competence and credibility

36.11 A witness is professionally competent if he/she may lawfully give evidence. Whether a witness is competent to give evidence as an expert is for the judge to determine.

36.12 A witness is credible if he/she is seen to be trustworthy, reliable and capable of being believed. Whether a witness is credible is for the jury to determine.

36.13 Whilst, for most purposes, the issue of competence and the issue of credibility of an expert may be kept distinct, there may be issues of credibility that affect the expert's competence, and vice versa. For example, an expert who has been convicted of a dishonesty offence may be regarded as less likely to be wholly objective and balanced in giving expert evidence.

36.14 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. It may be sufficient if the expert witness is employed by a reputable company or organisation and the expert's report cites relevant qualifications and experience. If the expert is a sole practitioner or a member of a small group, or the field of expertise is not widely recognised, more investigation may be required.

36.15 Investigators fulfil the duty to pursue all reasonable lines of enquiry by requiring expert witnesses, whose evidence is being relied upon, to reveal any information that might adversely affect credibility and/or competence as an expert witness.

36.16 Where an expert fails to reveal information that adversely affects credibility or competence, this may lead to:

  • a prosecution being halted or delayed;
  • adverse judicial comment;
  • convictions being found to be unsafe on appeal;
  • professional embarrassment;
  • disciplinary proceedings;
  • civil actions by an accused.

36.17 By way of example only, information that might undermine the competence and/or credibility of an expert witness might be:

  • the discovery that the expert has not used established procedures in a scientific process;
  • that scientific theories that have been applied have been discredited in the mainstream of the field of expertise;
  • that the expert has been partial in the information and material that has been taken into account in arriving at an opinion.

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The expert's Self-Certificate

36.18 The expert instructed should submit to the investigating officer or disclosure officer, a completed self-certificate (the 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.

36.19 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. A copy of this Certificate can be found in the Guidance Booklet at Annex K in this Manual.

36.20 This guidance does not apply to police employees They are required to complete an MG6B as set out in Chapter 18 of this Manual should they have any disciplinary and or criminal findings, or pending proceedings against them. Prosecutors are referred to Chapter 18 of this Manual for further guidance.

36.21 Failure by an expert to complete the Certificate may cause the prosecutor either to seek an expert who will, or to continue with that expert and disclose the information to the defence, if it meets the disclosure test.

36.22 Where an expert refuses to complete a Certificate, consideration will be given by the Prosecution Team to the use of the expert in future cases.

36.23 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.

36.24 Revealing information to the disclosure officer and the prosecutor does not automatically mean disclosure to the defence.

36.25 Where the expert has indicated that there is a positive answer to any of the questions on the certificate, the CPS prosecutor is responsible for considering whether the information satisfies the disclosure test.

36.26 The Certificate should be referred to on the MG6C and should not be sent to the defence. The Certificate should be described on the MG6C as: "completed Certificate by [name of expert]."

36.27 Any matters revealed on the Certificate that fall to be disclosed should be disclosed using the pro forma letter which can be found in the Manual at Annex C8.

36.28 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.

36.29 Prosecutors should in considering whether or not such material is deployable have regard to the authorities upon the proper extent of cross-examination, both as to an issue in the case and as to the witnesses' credibility, including, but not restricted to, the cases of R v Edwards (1991) 2 All ER 266; R v Guney (1998) 2 Cr App R 242 and R v Brooks [2002] EWCA Crim. 2107. A further example of the approach that ought to be taken to this issue is to be found in the case of R v Zomparelli CA 23 March 2000. (Unreported)

Bad character

36.30 In conjunction with paragraph 36.28 above prosecutors should also have regard to the provisions in 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 CPS legal guidance on bad character for guidance on whether this material can be used in court.

Unresolved complaints and disciplinary proceedings

36.31 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.


36.32 The expert must include in their statement a declaration (the 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 Booklet at Annex K.

36.33 Failure by an expert to complete the Declaration may cause the prosecutor to seek an expert, who will do so.

36.34 In addition where an expert refuses to complete the Declaration, consideration will be given by the Prosecution Team to the use of the expert in future cases.

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PNC checks

36.35 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 paragraph 11 of this Manual for further guidance.

Adverse judicial findings

36.36 An adverse judicial finding is a finding by a civil or criminal court, expressly or by inevitable inference that an expert witness has knowingly, whether on oath or otherwise misled the court.

36.37 With regard to experts this definition may be extended to include unintentional misleading of the court, i.e. incompetence.

36.38 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 who will pass the information to CPS Policy Directorate.

36.39 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.

36.40 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.

36.41 Policy Directorate does not retain a register of experts or a register of adverse judicial findings. However, Policy Directorate will circulate the information to Areas and Headquarters and provide guidance as appropriate. Policy Directorate will also inform the expert that an adverse judicial finding has been made against them and that the expert is therefore under an obligation to declare that to the Prosecution Team on future occasions on the Certificate.


36.42 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.

36.43 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.

36.44 If a doubt remains about whether disclosure is required, the prosecutor should disclose.

36.45 The decision to disclose or withhold information must be made or approved by a unit head, Special Casework Lawyer or equivalent.

36.46 Prosecutors should cross refer to other relevant chapters in the Disclosure Manual when necessary. In particular, prosecutors should refer to chapter 23: Disclosure of Unused Forensic Science Material.

Record of decision

36.47 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

36.48 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

36.49 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.

Guidance Booklet for Experts

36.50 The Guidance Booklet for experts can be found at Annex K and contains:

  • the Declaration for experts,
  • the experts Self-Certificate,
  • a specimen Index,
  • a flowchart of the experts revelation process.

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