CAT Proceedings and Participation Infrastructure: What Law Firms Get Wrong

The Competition Appeal Tribunal has not introduced a formal participation threshold into UK collective proceedings. No rule requires a proposed class representative to prove, in advance, how many claimants will engage. But recent certification decisions — most notably Waterside Class Limited v Mowi ASA & Ors (the Salmon proceedings) — show a Tribunal increasingly prepared to interrogate the practical assumptions underneath a claim: how many people will actually come forward, and whether the proceedings will deliver anything meaningful to the class once lawyers and funders have been paid.

This article looks at what Waterside actually said, why the reasoning matters more than the outcome, and what that means for how a CAT case’s participation strategy should be planned and evidenced.

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Key takeaways

• The CAT has not created a formal participation threshold — but recent certification reasoning shows operational assumptions attracting real scrutiny.
• Most collective actions don’t fail on law. They fail on participation.
• A CAT case’s participation strategy should be decided by whether the claimant population is already known — not by whether the case is opt-in or opt-out.
• Participation assumptions deserve the same evidence base as legal, economic and funding assumptions.

The Industry Is Asking the Wrong Question

Most conversations about claimant participation in CAT proceedings start with the wrong question: how do we recruit enough claimants? It is understandable, but narrow — it produces a media plan or an acquisition budget, treated as a discrete task that ends once registration numbers look healthy.

The better question is different in kind: can we build a claimant population capable of supporting the proceedings from certification through to settlement and distribution? That question extends across verification, years of case dormancy, and a distribution process that depends on the same population still being reachable when money is finally available. Answering it well starts at case design, not after certification.

Different questions produce different outcomes at certification. A recruitment question produces a campaign. A participation question produces a credible, evidenced answer to exactly the kind of scrutiny the Tribunal applied in Waterside.

Every CAT Case Depends on Participation

Every CAT collective action rests on a population of claimants who must be identified, informed, onboarded, retained and eventually brought to a point where they receive a remedy. It is useful to think of every collective action as resting on four load-bearing assumptions:

• Legal merit — is there a viable cause of action, properly pleaded and evidenced.
• Economic viability — do the likely damages justify the cost of pursuing them.
• Funding structure — is there capital in place, on terms that survive the life of the case.
• Participation capability — can a claimant population actually be built, retained and carried through to distribution.

Historically, the first three have received the most scrutiny — from lawyers, from funders, from the Tribunal itself. The fourth has largely been assumed. Waterside illustrates why that is changing.

[[ Insert Image block here: Figure 1 — The Four Foundations of Collective Proceedings (fig01-four-foundations.png). Alt text: “Diagram showing the four foundations of collective proceedings: legal merit, economic viability, funding structure, participation capability.” ]]

The Waterside (Salmon) Decision

On 15 April 2026, the Competition Appeal Tribunal handed down judgment in Waterside Class Limited v Mowi ASA & Ors [2026] CAT 32 — the Salmon proceedings. It is worth being precise about what the Tribunal did and did not decide.

It did not create a legal participation threshold, and it did not rule that a class representative must prove a minimum take-up before certification. What it did was decline, at this stage, to make a collective proceedings order — while leaving the door open to a revised application — because neither the eligibility nor the authorisation condition was satisfied on the material before it.

The claim was brought on behalf of an estimated 35–44 million UK consumers, alleging that six salmon producers had colluded to inflate prices between 2013 and 2019, with harm to the class put at £71–382 million. The Tribunal’s central concern was the relationship between proposed litigation costs — a budget of £15.75 million plus VAT, rising past £21 million once an After-the-Event (ATE) insurance premium was included, and disclosed only under questioning to potentially exceed £40 million with contingent success fees — and the amount likely to reach the class.

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£712,000–£3.1m — the Tribunal’s own projected return to the class, applying an illustrative 1% take-up rate, against a litigation cost base starting at £15.75m (source: Waterside Class Limited v Mowi ASA & Ors [2026] CAT 32).

Crucially, the Tribunal criticised the proposed class representative for failing to estimate claimant take-up at all. Applying that illustrative 1% uptake rate — drawn from its earlier Gutmann decision — it held that outcomes which predominantly benefit lawyers and funders rather than the class are not in the public interest. The claim was not struck out — a revised application with a reduced budget and better distribution mechanism was invited — but the reasoning is the point: a take-up assumption sat at the centre of whether the whole proceeding was viable.

Participation assumptions increasingly deserve evidence. Not optimism.

This should not be read as a suggestion that a formal participation test now exists — it is one carefully reasoned judgment on its own facts, and firms should take independent legal advice on its implications for any specific matter. What it illustrates is a sensible direction of travel: operational assumptions about participation are being read alongside legal and economic ones, not apart from them.

The Two Questions Every CAT Case Must Answer

Before any technology, campaign or team is resourced, every CAT matter needs an honest answer to two questions.

Do we already know who the claimants are? If not, how will we build the claimant population?

These questions matter more to a case’s operational design than whether it is opt-in or opt-out. A firm holding a defined set of business customers is in a fundamentally different position from one pursuing an opt-out claim for an unidentified population of millions, as in Waterside — the certification route may look similar, but the operational task, and the evidence needed to support it, is not.

[[ Insert Image block here: Figure 2 — The Two Participation Questions (fig04-two-participation-questions.png). Alt text: “Diagram of the two participation questions: do we already know who the claimants are, and if not, how will we build the population.” ]]

The Claimant Journey in a CAT Case

Every claimant who is ultimately part of a CAT class moves through a broadly similar journey, and every stage is a point where the population Waterside was concerned about can be lost: awareness, eligibility, registration, identity verification, evidence collection, retainer execution, validation, claimant portal access, ongoing communications, and settlement distribution.

Each stage needs a defined process, a measurable completion rate, and clear ownership of what happens when a claimant drops out. A claimant acquired at stage one and lost by stage four is the same participation failure as never having reached them — arguably worse, since the cost of reaching them has already been spent. It is exactly this kind of drop-off, multiplied across millions of class members, that produces the low take-up rates the Tribunal is now willing to scrutinise.

[[ Insert Image block here: Figure 3 — The Ten-Stage Claimant Journey (fig08-ten-stage-claimant-journey.png). Alt text: “Diagram of the ten-stage claimant journey from awareness to settlement distribution.” ]]

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Participation Confidence and Due Diligence

Legal assumptions are tested through pleading and judgment. Economic assumptions through damages modelling. Funding assumptions through due diligence before capital is committed. Participation assumptions — how many people will come forward, how they will be retained, whether they will still be reachable when it matters — have historically just been assumed.

Waterside shows why that gap matters: a take-up assumption determined whether an entire proceeding was viable. A case with high Participation Confidence can show its working at certification — historical take-up data on comparable claims, realistic reach modelling, a distribution mechanism stress-tested against the population it needs to serve. A case without it is asking the Tribunal, a funder or a client to take the same leap of faith the Waterside PCR was found not to have supported.

[[ Insert Image block here: Figure 4 — The Participation Confidence Model, illustrative (fig09-participation-confidence.png). Alt text: “Illustrative chart showing scrutiny historically applied to legal, economic and funding assumptions versus participation assumptions.” ]]

Participation due diligence is the discipline that gets a case there — examining, before certification, whether the population can realistically be reached, understood, onboarded, retained, and ultimately paid. Legal, economic and funding due diligence are established practices. Participation due diligence is not yet established in the same way — but Waterside suggests it should be.

[[ Insert Image block here: Figure 5 — The Participation Due Diligence Framework (fig10-participation-due-diligence.png). Alt text: “Diagram of the eight-question Participation Due Diligence Framework.” ]]

Conclusion

CAT proceedings have steadily professionalised legal practice and litigation funding over the past decade. Participation is now undergoing the same evolution, later than the other three foundations but no less consequentially. Waterside shows that operational assumptions increasingly matter alongside legal and economic ones — not because the Tribunal has introduced a formal participation threshold, but because the practical realism of a claim’s assumptions about its own class is becoming harder to separate from the merits of the claim itself.

Participation should be planned and evidenced as infrastructure from case design onward, not left to a communications plan assembled after certification. The firms that can show their working on participation, not just on law, will hold the advantage in the CAT proceedings that follow.

Most collective actions don’t fail on law. They fail on participation.