The company, which is a key player in the online ad market as well as being a dominant force in search, is accused of abusing its power in the ad tech market, which coordinates the sale of online advertising space between publishers and advertisers.

“Publishers, including local and national news media who play a vital role in our society, have long been harmed by Google’s anticompetitive conduct,” said Damien Geradin, of the Belgian law firm Geradin Partners which is involved in the EU case.

“It is time that Google owns up to its responsibilities and pays back the damages it has caused to this important industry. That is why today we are announcing these actions across two jurisdictions to obtain compensation for EU and UK publishers.”

The UK law firm Humphries Kerstetter is planning to bring a case to the Competition Appeal Tribunal over the next month, although the process could take years to reach a conclusion. The UK competition watchdog is also investigating Google’s power in the digital advertising technology market.

Toby Starr, a partner at Humphries Kerstetter, said the claim, which aims to recover advertising revenue lost due to Google’s allegedly anticompetitive behaviour over a period of years, would not just be aimed at benefiting news sites.

“This important claim will represent a class of victims of Google’s anti-competitive conduct in ad tech who have collectively lost an estimated £7bn. This includes news websites up and down the country with large daily readerships as well as the thousands of small business owners who depend on advertising revenue – be it from their fishing website, food blog, football fanzine or other online content they have spent time creating and publishing.”

The UK claim will be “opt out”, meaning that affected parties will be automatically treated as part of the claim, while the EU claim will be lodged in the Netherlands will be “opt in”, meaning would-be claimants need to apply to join the suit. Starr said he expected “many thousands” of parties in the UK to be part of the claim.

The combined suits are seeking total compensation that, according to estimates from legal representatives, could reach €25bn. The suits are being financed by litigation firms in the UK and the Netherlands, which take a cut of any proceeds from a successful case.

Contacted for comment, Google referred the Guardian to its statement on the Competition and Markets Authority (CMA) investigation, which said: “Advertising tools from Google and many competitors help websites and apps fund their content, and help businesses of all sizes effectively reach their customers. Google’s tools alone have supported an estimated £55bn in economic activity for more than 700,000 businesses in the UK and when publishers choose to use our advertising services, they keep the majority of revenue. We will continue to work with the CMA to answer their questions and share the details on how our systems work.