Executive summary
The High Court recently introduced formal guidance on the use of generative AI in High Court proceedings, including in expert reports (Practice Direction HC142 (the “Direction”)). Human oversight, verification and where applicable, disclosure is required when using generative AI (“GenAI”) in producing documentation for court proceedings. In the context of expert reports, the Direction reasserts that expert evidence must remain the expert’s own independent opinion.
Although the Direction does not specifically apply to tax appeals, it provides useful guidance on the use and perils of GenAI in expert evidence. The Direction came into force on 1 September 2026.
How does the High Court’s AI guidance affect Tax Appeals Commission proceedings?
While the Direction applies to High Court proceedings and does not, expressly, apply to proceedings before the Tax Appeals Commission (“TAC”), it will be prudent to follow the Direction if using GenAI in the course of TAC proceedings.
TAC hearings resemble court hearings, and parties may invite expert witnesses to give written and oral evidence. TAC determinations are final and conclusive[1], and appeals may be made to the High Court by way of case stated on a point of law.[2] Although the TAC has more flexible rules than the High Court on the admissibility of evidence, as the TAC is subject to judicial review, the procedures applied by the TAC in practice, tend to mirror those applied by the High Court. The Direction should therefore be taken into account when preparing for cases at the TAC. All lawyers should be aware of the Direction and the need to disclose the use of GenAI when preparing court and TAC documents. The Direction will also be relevant for expert witnesses providing reports at the TAC.
Expert evidence has become more prevalent in TAC determinations, arising from the growing complexity of disputes. There has been a marked increase in reliance on expert witnesses on accounting, economics and foreign law issues before the TAC. As the usage of GenAI increases, one would expect to see more experts relying on GenAI in the preparation of their reports.
What does the new guidance say about AI disclosure requirements and expert reports?
The Direction concerns the use of GenAI in the production of documents used in court proceedings. The scope of the Direction includes:
- All documents used in civil proceedings, including pleadings, submissions, affidavits, witness statements and expert reports (“Court Documents”);
- All persons in Court proceedings, including parties, legal practitioners, witnesses, experts and any third party required by the Court to produce documents;
- All proceedings before the Court whether instituted before or after its commencement; and
- All Court Documents prepared, or generated after 1 September 2026.
While legal practitioners will need to ensure that their use of GenAI is responsible and transparent, this update is focused on the use of GenAI in expert reports. The Direction requires that an expert report must reflect the deponent’s own opinion, expertise and reasoning processes. Expert reports should also contain a declaration to the effect that GenAI has not been used to generate the substantive content of the report and the report reflects the opinion, knowledge and evidence of the expert. This includes the use of GenAI in drafting, analysing or interpreting substantive content for the report.
If GenAI has been used, the expert must disclose the nature and extent of that use to facilitate the Court (or the TAC) in assessing the reliability, independence and evidential weight of the report.
Experts must provide independent assistance by way of objective, unbiased opinion on matters within their expertise and state the assumptions on which their opinion is based.[3] If experts use GenAI improperly, this may give rise to a breach of this duty and result in their evidence being disregarded.
Why are AI hallucinations and generative AI risks relevant to expert evidence?
The Direction identifies several risks associated with the use of GenAI in the preparation of court documents, including expert reports.
The Direction correctly highlights that GenAI is susceptible to inaccuracies. GenAI frequently produces material which may on its face appear credible, but in fact, generates fabricated information and fictitious authorities. This anomaly in GenAI systems is colloquially referred to as a “hallucination”. A notable instance of hallucinations occurring in the UK was in Cork & Anor v Smith [2026],[4] where a global law firm misled the Court twice by providing references which featured hallucinations and by using AI to produce an explanatory letter which was incorrect. This resulted in the firm self-referring to the UK’s Solicitors Regulatory Authority. This reflects not only the tendency of GenAI to hallucinate but also to inaccurately summarise genuine sources and contain incomplete information.
The case noted above identifies the real risks associated with the use of GenAI in Court Documents and may give rise to a breach of duty to the Court. All persons in Court proceedings, or TAC appeals, should exercise vigilance in using GenAI in light of such risks.
Several other jurisdictions have adopted similar practice notes or formal guidance on the use of GenAI in litigation proceedings, including in Australia and Canada.
Key takeaways
The Direction effectively identifies the risks associated with the use of GenAI in the production of court documents and expert reports.
Experts should be aware of their duty to the Court (and the TAC) in providing evidence. Evidence must be objective, unbiased and on matters within the expert’s expertise. If GenAI has been used substantially in their report, the expert must declare the extent to which it was relied upon.
[1] s949AP(1) TCA 1997
[2] s949AP(2) TCA 1997
[3] National Justice Compania Naviera SA v Prudential Assurance Company Ltd [1993] 2 Lloyd’s Rep 68, Duffy v McGee [2022] IECA 254
[4] [2026] EWHC 1199 (Ch)
