The UK Competition Appeal Tribunal issued a ruling Friday that closes a passive-membership loophole in the country’s largest platform-competition damages case: large corporate publishers automatically included in the £13.6B opt-out class action against Google must now actively produce documents as part of pre-trial disclosure.

The decision emerged from the third case management conference in Ad Tech Collective Action LLP v Alphabet Inc. & Others, which concluded May 21 and 22, 2026. It rejected the argument that passive class membership — the legal status that entitles tens of thousands of UK publishers to share in any eventual damages award without having individually signed onto the litigation — also exempts large institutional publishers from the evidentiary obligations that ordinarily accompany court proceedings.

The ruling drew a deliberate distinction between small, genuinely passive beneficiaries and large corporate publishers with the resources and organizational capacity to contribute to the evidentiary record. For the latter, the tribunal found, passive class membership is not a free pass.

Google Must Also Hand Over US and EU Enforcement Materials

Separately, the claimants secured a disclosure win against Google directly. The tribunal ordered the company to produce materials from the US Department of Justice’s ad tech proceedings, along with a less-redacted version of the European Commission’s September 2025 infringement decision — blocking Google’s attempts to roll back disclosure terms it had previously agreed to.

The Commission adopted that infringement decision on September 5, 2025, imposing a €2.95B fine — the second-largest antitrust penalty in EU history — after finding Google had unlawfully favored its own AdX ad exchange in both its publisher ad server and its ad-buying tools. Google has appealed that decision.

How Google’s Ad Stack Allegedly Took Revenue From Publishers

The case’s central allegation rests on a specific architectural conflict in Google’s ad-tech stack. When Google acquired DoubleClick in 2008, it gained control of both DoubleClick for Publishers — the dominant publisher ad server used by large websites to manage and auction their advertising space — and the DoubleClick Ad Exchange, known as AdX. Those two products were merged in 2018 into what is now called Google Ad Manager.

The problem, according to regulators who have now ruled against Google in the United States and at the European Commission, is that Google used control of both layers simultaneously. A publisher ad server acts as the technology that decides which exchange wins each ad impression. When Google owned both the referee and one of the teams in every auction, it could give its own exchange advantages that competing exchanges could not match.

According to the European Commission’s decision, DFP informed AdX in advance of the value of the best competing bid — giving AdX what practitioners have called a “Last Look” advantage before the auction concluded. In practice, this meant Google’s exchange did not compete on equal terms with rival exchanges; it entered each auction already knowing what it needed to beat.

In April 2025, US District Judge Leonie Brinkema confirmed in United States et al. v. Google LLC that Google had monopolized two markets — the publisher ad server market and the open-web display ad exchange market — and had illegally tied the two together. The court found that Google “harmed Google’s publishing customers, the competitive process, and, ultimately, consumers of information on the open web.”

For UK publishers included in the class — estimated at around 130,000 businesses across roughly 1.75 million websites and apps — economic analysis submitted with the claim suggests Google’s practices may have reduced their advertising revenues by as much as 40% over the claim period.

Google Faces Coordinated Global Enforcement

Friday’s disclosure ruling is one procedural step in a case that now sits alongside two of the most significant platform-competition enforcement actions in the world.

The European Commission’s September 2025 decision found that Google had violated Article 102 of the Treaty on the Functioning of the European Union by unlawfully favoring AdX in both its publisher ad server and its ad-buying tools. In the United States, the DOJ’s April 2025 liability ruling established that Google had violated the Sherman Act by monopolizing open-web digital advertising markets. A remedies trial concluded with closing arguments in November 2025; Judge Brinkema’s ruling on whether to order structural relief — potentially including a forced sale of AdX — remains pending as of publication.

The UK class action trial is listed for September 2028, with a 12-week hearing scheduled to determine whether Google’s conduct caused the alleged harm to British publishers.

Why This Ruling Matters for Future UK Class Actions

The tribunal’s willingness to draw a line between small passive beneficiaries and large institutional class members carries implications beyond the Google case itself. The opt-out collective proceedings regime, introduced under the Consumer Rights Act 2015, was designed to lower barriers to antitrust redress — allowing affected parties to benefit from litigation without individually participating.

Friday’s ruling suggests that regime’s passive-membership protections are not absolute. Law firms and litigation funders running future opt-out collective actions will need to account for the possibility that large corporate class members may face disclosure obligations whether or not they played any active role in launching the claim.

Google has contested the case at every stage, having unsuccessfully challenged certification before the CAT in June 2024, sought permission to appeal at the Court of Appeal in January 2025, and continued to challenge the scope of its own disclosure obligations.

What Is the £13.6B Google Ad Tech Claim?

Ad Tech Collective Action LLP — led by former Ofcom director Claudio Pollack, technology journalist Charles Arthur, and competition lawyer Kate Wellington — is represented by Hausfeld & Co. LLP, Humphries Kerstetter LLP, and Geradin Partners. The litigation is backed by third-party funding, meaning UK publishers automatically included in the class bear no direct financial risk from participating. The claim period covers January 2014 to November 2022.

Frequently Asked Questions

What does the CAT’s disclosure ruling mean for publishers in the Google ad tech class action?

For most publishers — small websites and apps included in the opt-out class — nothing changes. The ruling targets large corporate class members: institutional publishers with the resources and organizational capacity to produce relevant documents. Those publishers can no longer treat the funded lawsuit as a cost-free windfall that requires no engagement. For smaller class members, the litigation continues to be conducted entirely on their behalf by Ad Tech Collective Action LLP.

What did Google actually do wrong in its ad-tech business?

According to rulings by the US Department of Justice and the European Commission, Google used its simultaneous control of two critical parts of the advertising supply chain — the dominant publisher ad server (DoubleClick for Publishers, now Google Ad Manager) and the dominant ad exchange (AdX) — to give its own exchange structural advantages over competitors. The EC’s decision found that DFP informed AdX in advance of the value of competing bids, allowing Google’s exchange to win auctions it should not have won on a level playing field. A US federal court confirmed in April 2025 that Google illegally monopolized both the publisher ad server market and the ad exchange market, and illegally tied the two products together.

Can UK publishers still join or leave the £13.6B class action?

The opt-out period closed on May 28, 2025. UK-domiciled publishers and app operators who received advertising revenue through Google’s systems between January 2014 and November 2022 are automatically included in the class unless they actively opted out before that deadline. Publishers who missed the deadline and wish to opt out should write directly to the Competition Appeal Tribunal to seek permission. The case is funded by third-party litigation funders, meaning class members bear no financial risk.

When will the UK Google ad tech case reach trial?

The Competition Appeal Tribunal has listed the trial for September 2028, with a 12-week hearing to follow. Before then, the parties must complete disclosure and exchange expert and factual witness evidence under the timetable set in February 2026. Friday’s disclosure ruling is part of that pre-trial preparation.