CAT issues new Practice Direction on expert evidence

What you need to know

• The CAT published a new Practice Direction in relation to expert evidence on 2 December 2025.  
• In light of recent experience, the CAT is making changes to the expert evidence process in order to improve efficiency and efficacy.

On 2 December 2025 the Competition Appeal Tribunal (CAT) published a new Practice Direction in relation to expert evidence. The Practice Direction sets out the CAT's new expectations in relation to all expert disciplines, but with specific emphasis on principles governing economic evidence. 

The Practice Direction covers a number of key issues for expert evidence based on the CAT's experience in recent cases, including:

• More active case management. The CAT now expects parties to identify experts and the issues they will address at an early stage in order to avoid duplication. In some cases, the CAT may require parties to instruct a joint expert, where they have aligned interests on a set of issues. At an early stage experts will also be expected to disclose and justify their methodologies to be relied upon at trial, with reference to the pleaded cases. As the case progresses, experts' request for disclosure must be narrow and targeted.  
• An emphasis on experts' independence. The CAT expects experts to take personal responsibility for their work, approaching issues in a fair-minded manner and updating their views where appropriate in light of new evidence. In addition, experts must disclose prior involvement with the client or the case when seeking permission for expert evidence or with any report filed without prior permission (e.g., at CPO stage). Non-disclosure may result in the evidence being disregarded.
• Improving the readability of expert reports. The CAT now expects 'plain-English' expert reports that can be understood by non-experts and experts should avoid lengthy background explanation (with page limits at the CAT's discretion). Where technical material is absolutely necessary it should be confined to an annex. 
• Clear expectations of experts' approaches. The new Practice Directions encourage experts to undertake empirical analysis that is applied to the facts of the case, and avoid 'purely theoretical' evidence. Expert evidence should be supported by established sources such as textbooks, major authority guidance and widely cited peer-reviewed literature. Experts should engage with relevant CAT and UK case law, and coherently explain where their methodology has departed from standard approaches.

Whilst it remains to be seen how the Practice Direction will be implemented in individual cases, it provides welcome guidance in how expert evidence should be adduced. It is clear that the CAT is unhappy with how expert evidence in some recent cases has ballooned in volume and complexity and experts will be under significant pressure to ensure their evidence is concise, transparent and fair-minded.

One issue that remains to be determined is how experts will manage the need to explore complicated issues at an early stage in the proceedings. This is sometimes addressed through the use of so-called 'advisory' experts who are kept separate from the team of the testifying expert. The role of an advisory expert is likely to become more important as clients navigate the requirements that the new Practice Direction places on their testifying expert.  

Ashurst's Antitrust Litigation team has for years included a team of highly experienced in-house economists who work with our lawyers and alongside the independent (testifying) expert to assist in this advisory role.  If you would like to discuss our capabilities and experience of litigating in the CAT further with a member of our team, please get in touch.