Google is the U.K.’s first big antitrust target after the country quit the European Union as regulators opened a probe Friday into the company’s planned changes to advertising data.

The Competition and Markets Authority said it’s “investigating Google’s plans to remove third-party cookies and other functionalities from its Chrome browser,” according to an emailed statement. This could “undermine the ability of publishers to generate and undermine competition in digital advertising, entrenching Google’s market power.”

Publishers and advertising technology companies complained in November that Google’s so-called privacy sandbox changes will curb members’ ability to gather information on web users, which helps them offer more valuable advertising. Smaller media companies are at risk of losing as much as 75 per cent of their revenue, it said.

Google upended the advertising world with its decision last year to phase out third-party cookies that help advertisers pinpoint customers with ads for websites they previously visited and monitor which ads convinced them to buy. Google’s Chrome is used by the overwhelming majority of internet users and the changes will be followed by browsers based on Google technology such as Microsoft Corp.’s Edge.