Google must face a £13.6 billion ($17.4 billion) class action brought on behalf of website publishers which run advertisements over alleged anticompetitive practices, Britain's antitrust tribunal said Wednesday.

The Competition Appeal Tribunal has granted permission for Ad Tech Collective Action LLP, a special purpose vehicle created to represent the publishers, to bring collective proceedings against Google LLC and its parent company, Alphabet Inc.

Judge Marcus Smith, leading the three-strong panel, rejected Google's argument that the claimants' case was not sufficiently pleaded. Judge Smith added that the claimants' expert witness could have better explained the difference between the "real world" and a counterfactual one in which the market operated indiscriminately.

But he found Ad Tech's "counterfactual to have been sufficiently pleaded for Google to know the case it has to meet."

The massive claim was originally two separate opt-out collective actions brought by Charles Arthur and Claudio Pollack, who set up Ad Tech, which were consolidated into one by the CAT in October in a landmark decision.

The parallel claims by Arthur, a British journalist, and Pollack, a former director of the Office of Communications — the communications watchdog Ofcom — alleged that Google's anticompetitive actions had caused rivals to lose hundreds of thousands of pounds in profits since 2014.

Google's stranglehold on the U.K.'s ad tech market has allowed the technology giant to unlawfully restrict competition by favoring its own services, the claims allege. This has caused rivals to lose up to 19% of the revenue they would have otherwise received since 2014, they claim.

The tribunal also concluded that Ad Tech's methodology for proving that any damage was caused by Google's alleged abuse of its market dominance is sufficient for the case to go to trial. The CAT might decide at trial that only some of the alleged abuses were made out, "and it would be necessary to have a methodology robust enough to deal with that outcome," according to the decision.

But the panel — on which Judge Smith was joined by lay members Maria Maher and John Alty — concluded that Ad Tech and its expert had considered that scenario and that it is not necessary to state "each and every combination of failure or success" at trial in the pleadings or expert report.

"If that was Google's contention — and at times it appeared to be — then we reject it as both contrary to law and oppressive to the class' access to justice," the CAT wrote.

The panel observed that Google "came very close to suggesting that a claim should only proceed if a prima facie articulation of loss could be produced." That approach would go further than the tribunal legal test requiring a claimant to set out a sufficient blueprint for trial "and act as a barrier to justice by requiring a claimant to meet an unrealistically high threshold for the articulation of their methodology."

The CAT found that the allegations made by the claimants are triable, and that the harm to the class and damage suffered by it can be quantified at trial.

The panel said it did not consider Google's points that some of the claims were time-barred, because they are "not preclusive" of a collective proceedings order being granted. The panel concluded that the issues of limitation and strike-out should be dealt with in the main trial.

"Any other course would involve, for no good reason, disrupting the process to trial," the CAT said.

Pollack said in a statement that the decision to certify is "of major importance to the victims of Google's anticompetitive conduct in ad tech.

"I look forward to working with our legal and economic advisers to deliver compensation for years during which the relevant markets did not provide a competitive outcome for the UK publishing market," he added.

Google did not immediately respond to a request for comment.

Ad Tech Collective Action is represented by Robert O'Donoghue KC of Brick Court Chambers, Gerry Facenna KC, Julian Gregory and Nikolaus Grubeck of Monckton Chambers and Greg Adey of One Essex Court, instructed by Humphries Kerstetter LLP, Hausfeld LLP and Geradin Partners Ltd.

Google was represented by Meredith Pickford KC of Monckton Chambers, Connall Patton KC of One Essex Court and Natasha Simonsen and Warren Fitt of Blackstone Chambers, instructed by Herbert Smith Freehills LLP.

The case is Ad Tech Collective Action LLP v. Alphabet Inc. & Others, case numbers 1572/7/7/22 and 1582/7/7/23, in the Competition Appeal Tribunal.