The expert

Specialist inspectors

1. Specialist inspectors might be involved in an investigation either as part of the investigation team (albeit with specialist knowledge) or as an independent expert witness. The specialist knowledge of inspectors involved in an investigation may allow them to be treated as an expert witness in the event of a prosecution and thus give opinion evidence. The courts have held that the fact that an expert may be employed by one of the parties to litigation or the investigating authority would not debar them from giving expert evidence 1. The same principle applies to inspectors acting as experts in HSE prosecutions.

2. There is no rule which requires an expert witness to be independent of the investigation. Although a specialist inspector will not usually be appointed as an independent expert if they are closely involved in the investigation as to do so might impact on the weight to be attached to their evidence. The decision as to whether an 'independent expert' is required, and who it should be, rests with the person responsible for managing the investigation or prosecution on behalf of HSE usually based on legal advice, not the specialist. It is something to be decided on a case by case basis taking into account various factors including how closely the specialist worked alongside investigators, the nature of their evidence and the way a prosecution case is to be put. HSE has successfully prosecuted many cases without having to instruct an external expert or a further in house specialist unconnected with the investigation.

3. Whilst a specialist inspector may advise the investigator/s on the gathering of evidence, such as questions to put to witnesses or to a suspect, if a specialist has been appointed (or is intending) to act as a potential expert in any future proceedings they should generally avoid taking statements as part of the investigation or attending an interview of a suspect under caution (PACE interview) if possible. If a specialist inspector (who is not to act as an expert) attends a PACE interview, the fact of their specialist expertise should be disclosed.

The role of an expert

4. Expert witnesses enjoy three privileges over ordinary witnesses of fact:

  • They may remain in court prior to giving evidence, subject to argument about disputed factual evidence they may give, in order to hear evidence of both fact and circumstance. The permission of the court should be sought out of courtesy;
  • They may express an opinion on matters within their field of expertise; and
  • They may refer to the works of others within their field of expertise without infringing the rules against hearsay - see the [Hearsay section].

5. An expert witness may give opinions to assist in resolving issues concerning matters of knowledge which can only be acquired by special training or experience 2. Their role is to assist the court or jury on matters where their ordinary, everyday experience does not enable them to adequately consider the issues in the case 3. It is for the court to decide whether a witness is properly qualified to give an expert opinion 4 and the admissibility and weight of their evidence 5. Factors which the court may take into account when determining the reliability of expert opinion are detailed in the Criminal Practice Directions 2015; they include the quality of data, the validity, accuracy and reliability of methods used, safety of inferences relied upon, peer review, range of expertise, completeness of information available and range of opinion. 6

6. The facts on which the expert's opinions are based must be either admitted or proved by admissible evidence. Usually, such facts are established by other witnesses but they may include facts observed by the expert him/herself.

Duties of an expert

7. An expert's opinion must be objective and unbiased. Although a defendant will be entitled to call their own expert evidence it is the duty of an expert witness instructed by the prosecution (and indeed an expert witness called by any defendant) in a criminal case to act in the cause of justice. An expert's duty to the court overrides any obligation to the party instructing or paying them and includes an obligation to define their area/s of expertise in their report and when giving evidence in person. The expert must assist the court by giving objective, unbiased opinion within their area of expertise 7 whilst complying with the rules of evidence.

8.They must actively assist the court in fulfilling its duty of case management, in particular by complying with court directions and informing the court of any significant failure to take a necessary step in that regard.8 The expert must inform all parties and the court if their opinion changes from that contained in a report or statement served in evidence. An expert's report must comply with CrimPR 19.4 and contain a declaration substantially the same as provided in paragraph 19B of the Criminal Practice Directions 2015 Amendment No. 2 (CPD). The wording of the declaration is provided in Appendix 1.

9. The responsibilities in relation to expert evidence are shared between the expert and the prosecutor, but it is the prosecutor who has overall responsibility for the case; the expert is expected to assist with that task.

10. The duties and responsibilities of expert witnesses in criminal cases include the following:-

  • To read and ensure compliance with Part 19 of the Criminal Procedure Rules, the Code of Practice for Experts, the entitled booklet Guidance Booklet for Experts: Disclosure: Experts' Evidence, Case Management and Unused Material
  • Expert evidence presented to the court should be (and be seen to be) the independent product of the expert and should not be influenced as to form or content by the demands of litigation;
  • An expert witness should provide independent assistance to the court in relation to matters within his or her expertise. An expert witness should never assume the role of an advocate;
  • An expert witness should state the facts or assumptions upon which their opinion is based. They should consider all material facts in drawing conclusions and should not omit to consider those that detract from their conclusions;
  • An expert witness should make it clear when a particular question or issue falls outside their expertise. The opinion of an expert is only admissible if they are competent (ie qualified), through study, training or experience, in a field relevant to the case;
  • If the opinion provided by an expert is ,for some reason ,provisional egĀ  the data provided to the expert is insufficient to form a final conclusion, the report should be marked 'PROVISIONAL' and the reasons why provided;
  • If for any reason eg after exchange of reports with the defence, an expert changes their view on a material matter, such change of view should be communicated to the prosecutor and disclosed to the defence (via the prosecutor) without delay and, when appropriate, to the court;
  • If an expert considers that insufficient data is available to allow them to come to a final conclusion, this must be stated with an indication that the opinion is a provisional one;
  • If, after exchange of reports, an expert witness changes their view on a material matter (having read the defence expert's report or for any other reason),such change of view should be communicated to the defence (via the prosecutor) without delay and, when appropriate, to the court;
  • Where expert evidence refers to photographs, plans, calculations, analyses, measurements, survey reports or other similar documents, these must be provided to the defence at the same time as the exchange of reports.
  • To comply with the their duties to record, retain and reveal material in accordance with the Criminal Procedure and Investigations Act 1996 (as amended).

11. Experts who are civil servants must also carry out their duties in accordance with the Civil Service Code and its core values of integrity, honesty, objectivity and impartiality. The Code reinforces the standards expected of all experts by the courts.

Instructing an expert

12. It is the responsibility of the person managing the investigation to manage the use of specialists, deciding how they should be used and ensuring that experts remain independent. There is guidance in the Use of Specialists - Specialist Assistance Procedure (in Operational Procedures).

13. Any correspondence with an expert pertaining to their instructions will be disclosable and the defence will be entitled to see it. Whilst instructions to an expert explaining what is being asked of them will normally be recorded in formal correspondence (and see also the section 'The report' paragraph 6), the defence will also be entitled to see any "informal" correspondence. This applies to the instruction of in house specialists as it does to external experts and so care should be taken when drafting all communications with specialist and experts generally. For further guidance, see the paragraphs on expert evidence in the section 'Approach to common categories of material' in Expert and Specialist.

14. When an expert witness is instructed, it is important that they understand what is required of them. The expert should be referred to the following:

15. The expert must fully understand that they have an overriding duty to assist the court and should not feel prevented from providing information that might prove detrimental to the prosecution case. In order to meet that overriding duty, they are under an obligation to assist the prosecution with the statutory requirements relating to disclosure. The expert should be reminded of this obligation, which takes precedence over any internal codes of practice or other standards set by professional organisations.

16. Those instructing an expert will need to think carefully about what information should be supplied to them. Experts enjoy particular privileges arising from their qualifications and expertise. Those privileges, in particular the ability to put opinion evidence before the jury, can be undermined if it appears that their opinion has been improperly influenced. Any material provided to the expert should be listed in the original and any subsequent instructions. The expert should not be given material (such as internal prosecution or investigation reports) which is likely to contain opinion evidence from people other than witnesses of fact or other experts unless there is specific legal advice on the point. The defence is entitled to see all material provided to the expert by those instructing them and therefore sensitive or privileged material must not be supplied.

17. Experts should not be asked to conduct tests that will only favour the prosecution. The Criminal Procedure and Investigations Act 1996 requires investigators to pursue all reasonable lines of enquiry, and this means that tests that might undermine the prosecution case should also be commissioned.

18. Responsibility for directing the investigation remains with those instructing the expert. If the expert believes that further work or information is required, or if they change their evidence, they should communicate this to the person managing the investigation. Discussions with experts should be recorded, as they are clearly relevant to the investigation (and disclosable), particularly any discussion in which additional information is provided to the expert.

19. When an expert report is served on another party or the court, those instructing the expert must inform them at once. 9

20. The court may direct that experts acting for the parties should discuss the expert issues in the proceedings and prepare a statement for the court on the matters on which they agree and disagree, giving their reasons.10 Save for the statement itself, the content of the discussion must not be referred to in court without the court's permission 11.

21. Experts should be reminded that a failure to comply with their duties or a direction of the court could have a number of adverse consequences, including the delay or halting of the prosecution case, exclusion of the expert evidence, the overturning of any conviction and criticism of the expert by the judge, which might result in referral of the expert to any relevant professional body. Such consequences might prevent him/her from acting as an expert in future.

22. If questions remain about the role or instruction of experts, advice should be sough from Legal Advisors Office or an appointed solicitor agent.


  1. Field v Leeds City Council ([1999] C.P.L.R 833); R v Gokal [1999] 6 Archbold News 2, CA. See also R v Stubbs [2006] EWCA Crim 2312. Back to reference of footnote 1
  2. Folkes v Chadd (1782) 3 Doug KB 157. Back to reference of footnote 2
  3. R v Turner [1975] 60 Cr. App. R. 80, CA. Back to reference of footnote 3
  4. R v Silverlock [1894] 2 QB 766. In R v Oakley (1979) Crim LR 657 CA, a police officer who had attended a course, passed an exam as an accident investigator and attended more than 400 accidents was entitled to give expert evidence as to the cause of an accident. HSE inspectors should be permitted to give such evidence. Back to reference of footnote 4
  5. R v Kevin John Hodges & Another [2003] EWCA Crim 290, R v Dlugosz and Others [2013] EWCA Crim 2. Back to reference of footnote 5
  6. Criminal Practice Directions Amendment No. 2 [2014] EWCA Crim 1569. Back to reference of footnote 6
  7. Rule 19.2 (1)(a) Criminal Procedure Rules ('CPR'). See also R v B [2006] EWCA Crim 417. Back to reference of footnote 7
  8. Rule 19.2 (1)(b) CPR. Back to reference of footnote 8
  9. Rule CPR. Back to reference of footnote 9
  10. Rule 19.6 (1) and (2) CPR. Back to reference of footnote 10
  11. Rule 19.6 (3) CPR. Back to reference of footnote 11

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Updated 2023-11-13