A lawsuit was filed against Google to seek 3.4 billion $4.2 billion in compensation for publishers who lost revenue.
The claim, by ex-Guardian technology editor Charles Arthur, alleges that Google unlawfully used a dominant position in online adverts in a way that reduced what publishers could make from them.
Google would fight the speculative and opportunistic action vigorously.
It is the second such lawsuit after a similar case was launched in November.
The former Ofcom executive Claudio Pollack brought in the lawsuit, which is looking for up to 13.6 billion dollars in damages from the tech giant.
Advertising technology adtech decides in a fraction of a second what online ads consumers will see, how much they will cost, and how much publishers will earn.
Online display advertising is the main source of income for many websites.
Competition and Markets Authority CMA, the Competition and Markets Authority, is investigating Google's dominance in advertising technology.
In the lawsuit filed on Thursday, Arthur claims that publishers' ads revenues were unlawfully reduced because of Google's abuse of its position.
The CMA is investigating Google's anti-competitive conduct in adtech, but they don't have the power to make Google compensate those who have lost out. He wrote that we can only right that wrong through the courts, which is why I am bringing this claim.
Both legal claims ask the Competition Appeal Tribunal to certify their claims as opt-out, meaning that every relevant publisher would be automatically included in the case unless they choose otherwise.
Collective claims, often referred to as a class action in the United States, were only possible in the UK in 2015. Damages can be very large because they are brought on behalf of a whole group or class.
Unless Mr Arthur and Mr Pollack agree to collaborate, the tribunal will have to decide which one should lead the collective claim.
Google has said its advertising tools and those of its many Adtech competitors help millions of websites and apps fund their content, and enable businesses of all sizes to reach new customers. The CMA found that Google has the largest provider in three key areas of adtech, but the firm maintains that it has many competitors. It says its adtech fees are lower than industry averages.
In January, the US Justice Department accused Google of being an industry giant that has corrupted legitimate competition in the adtech industry by engaging in a systematic campaign to seize control of the wide swath of high-tech tools used by publishers. Google asked a court to dismiss the case, arguing that the US government had overstated its hold on the market.
In 2021, the French competition regulator, Autorit de la concurrence, fined Google €220 million for favouring its own services in the online advertising sector.