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Articles Health & Safety 12th Jun 2025

ALERTER
An essential guide to the admissibility of expert evidence in regulatory prosecutions

By Tim Green KC & Christopher Adams

Click here to download this Alerter by Tim Green KCChristopher Adams.

There have been a number of well-publicised recent collapses of complex health & safety prosecutions, with significant cost implications for the prosecution and court system.  A common theme has been a failure to understand the obligations on a party seeking to admit expert evidence.  In the light of these cases, this article is intended to give busy practitioners an essential guide to the statutes, guidance, rules and case law governing expert evidence and subsequent disclosure.

IS EXPERT EVIDENCE ADMISSIBLE?

To a large extent, the rules governing the admissibility of expert evidence in criminal proceedings are identical for the prosecution and the defence.  Opinion evidence is only admissible in criminal proceedings if it comes within strictly defined criteria.  Expert opinion evidence will only be admissible if it is given upon a subject calling for expertise, which a lay person could not be expected to possess to a degree sufficient to understand the evidence given in the case unaided.  Expert evidence is inadmissible if the tribunal of fact can form its own opinion without the assistance of an expert, because it is unnecessary (Turner [1975] QB 834 at p.841).  A lack of formal qualifications should properly be considered and made clear, but is not determinative of whether a person is entitled to give expert opinion (Byrne [2021] EWCA Crim 107).

In the context of health and safety work, an expert’s opinion would generally be admissible as to, for example, the necessary conditions for the safe operation of a complex manufacturing process.  On the other hand expert evidence would not, in our view, be generally admissible on the question of whether, for example, a Principal Contractor, Designer or other relevant person had actually been appointed pursuant to the Construction Design and Management Regulations.  That is because the question of whether a person has been appointed to fulfil such a role (as opposed to what such a role might generally involve) is a matter of fact for a jury.

This rule governing the limited scope to admit expert evidence applies to both prosecution and defence.  Practitioners should have it in mind at an early stage.  Building a prosecution or defence case around expert evidence which may later be ruled inadmissible during a trial carries obvious risks.

THE CRIMINAL PROCEDURE RULES 2020

If the issue is one which does genuinely call for expert opinion evidence in order for it to be understood, then the Criminal Procedure Rules (CrimPR) require the instructed expert to give an opinion which is both (i) objective and unbiased, and (ii) within the expert’s area or areas of expertise (Part 19.2(1)(a)).

In the same vein, a party who wants to introduce expert evidence otherwise than as admitted fact must serve with their expert report, among other things, notice of anything of which the party serving it 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 (Part 19.3(3)(c)(i)).  This obligation to transparency and objectivity is onerous.  It can include earlier drafts of an expert report where a different opinion was expressed by the same expert, and published research going to questioning or undermining the expert’s opinion.

An expert’s report must, among other things, contain a statement setting out the substance of all facts given to the expert which are material to the opinions expressed in the report, or upon which those opinions are based.  This includes any information the court may need to decide whether the expert’s opinion is sufficiently reliable to be admissible as evidence (Part 19.4(c) and (h)).  An expert’s opinion based only on a partial summary of the relevant facts will be badly undermined.  The disclosure obligation will also include disclosing the expert’s instructions.

CRIMINAL PRACTICE DIRECTION 2023

Case law and the CrimPR are then supplemented by section 7 of the Criminal Practice Direction (“CrimPD”) which emphasises again that for expert opinion evidence to be admissible: (i) it must be relevant to a matter in issue in the proceedings; (ii) it must be necessary  to provide the court with information likely to be outside the court’s own knowledge and experience; (iii) the expert witness must be competent to give that opinion; and (iv) the expert opinion must be sufficiently reliable to be admitted (7.1.1.a-d).

There are a wide range of factors which the court may take into account in determining the reliability of expert opinion, which are likely to be relevant to both admissibility of the evidence and, if it is admitted, the weight the jury might attach to it.  These factors include the extent to which the expert’s opinion is based on material falling outside the expert’s own field of expertise, the completeness of the information available to the expert, and whether the expert took account of all relevant information in arriving at the opinion (including information as to the context of any facts to which the opinion relates) (7.1.2.f-g).

In addition practitioners need to have in mind that, even assuming the expert’s opinion is admitted, the Judge giving his legal directions to the jury will have to identify potential flaws in the expert’s opinion which detract from its reliability.  For example, if it is based on a hypothesis which has not been subjected to sufficient scrutiny or which has failed to stand up to scrutiny; based on unjustifiable or questionable assumptions; relies on flawed or incomplete data; relies on an examination, technique, method or process which was not properly carried out or applied, or was not appropriate for use in the particular case, or relies on an inference or conclusion which has not been properly reached (7.1.3.a-e).

Fully discharging the disclosure duty

As will be obvious from the paragraphs above, the duty to fully disclose any relevant material which might undermine an expert’s opinion is onerous.  Furthermore, and in order to enable a full and complete assessment of the reliability of any expert evidence relied upon, all potentially relevant information must be disclosed both in relation to the expert and in relation to any corporation or other body with which the expert works.  Non-exhaustive examples of this wider disclosure duty include: (i) any fee arrangement under which the amount or payment of the expert’s fees is in any way dependent on the outcome of the case; (ii) potential conflicts of interest, and (iii) adverse judicial comment regarding a particular expert or corporation or other body for whom the expert works, whether by a first-instance tribunal or on appeal (7.1.4.a-c).

Where matters ostensibly within the scope of the disclosure obligations come to the attention of the court without having been disclosed, then this failure can lead to the expert’s opinion being excluded.  Once a trial has started, shortcomings in disclosure may not be easily rectified.  If disclosure failings make a fair trial impossible because the Judge and jury cannot properly assess the accuracy and reliability of expert evidence, such evidence is likely to excluded (for example under s.78 or s.81 PACE 1984 or s.20 CPIA 1996) (see para 7.1.6).

CPS EXPERT GUIDANCE ON DISCLOSURE, UNUSED MATERIAL AND CASE MANAGEMEMNT

Section 4 of the CPS Expert Guidance on Disclosure, Unused Material and Case Management underlines these duties as they apply to prosecutors.  Prosecution experts should also have regard to information that points away from, as well as towards the suspect (4.6), retain material, keep records of all work carried out and any findings made in relation to the investigation, and reveal unused material to the Prosecution Team to enable them to make informed decisions as to disclosure (4.8 – 4.10).

Unused material which is potentially disclosable to the defence on the basis that it assists the defence case or undermines the prosecution case might include, among other things, any draft report prepared by a prosecution expert, and material that has a bearing on the competence or credibility of an expert witness.  Where a prosecution expert’s report requires clarification, the guidance makes clear that any discussion with the expert by telephone may need to be added to the Disclosure Schedule.  Where a case conference takes place, there needs to be a clear, auditable record of the expert’s original view, including for disclosure purposes.

DUTIES OF LAWYERS IN RELATION TO EXPERT EVIDENCE

In general, lawyers instructing expert witnesses should satisfy themselves that the experts have the necessary expertise (Pabon [2018] EWCA Crim 420 at [77]).  It is also the lawyers’ responsibility to make sure that experts understand the requirement of the CrimPR and CrimPD, and that expert reports served are reliable and admissible (DPP v Walsall Magistrates’ Court [2019] EWHC 3317 (Admin)).

COMMENT

For those on the prosecution side, a failure to comply with the rules relating to expert evidence can have devastating consequences, including prosecutions collapsing and, in sufficiently extreme circumstances, orders to pay defence costs.  On the other hand, for those acting for the defence, an early appreciation and highlighting of deficiencies in prosecution expert evidence may reap dividends at trial, whether in confining the prosecution case within sensible limits, or (where expert evidence is irretrievably undermined) leading to the prosecution offering no evidence, or (in rare circumstances) resulting in an order for the prosecution to pay defence costs.

From a tactical perspective, prosecutors must be prepared to give robust and early advice to their clients if they consider that there are failings in the expertise, reliability or disclosure relating to a particular expert witness.  Defence lawyers need to rigorously assess prosecution expert evidence against the rules and guidance above and, where the standards are not met, draw this to the attention of the prosecutor before trial (e.g. in a defence statement) if they want the ear of the Judge at trial.  Taking a disclosure argument late, without having given the prosecution a fair opportunity to respond, will not impress the Judge.  It makes it more likely the prosecution will be given time to address any deficiencies, and may even rebound on the defence leading to costs being thrown away if delays and an adjournment could reasonably have been avoided by timely disclosure of the defence case on inadmissibility.

 

Tim Green KC
Christopher Adams
12 June 2025

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