Britain's market watchdog is examining plans to block Google for two years or more from rolling out new privacy features on its Chrome internet browser.
The Competition and Markets Authority is weighing up proposals to put the update on ice over fears that it could crush advertising revenues at smaller rivals.
The regulator has already launched an investigation into the technology after campaigners warned it could harm other websites by robbing them of the data they need for advertising.
Concern: Google claims the new features will protect internet users' privacy
Google claims the new features will protect internet users' privacy, even though the tech giant will still be able to use similar data itself. Its 'sandbox' privacy settings will stop smaller websites from using cookies to collect data about visitors' internet activity. Advertisers rely on cookies to target people with offers they think they will like.
Pressure group Marketers for an Open Web has told the CMA the project could hit smaller websites' revenues by up to 75 per cent.
The CMA launched its probe on January 8, but The Mail on Sunday understands its investigation could take two years or more.
Officials fear any changes to the browser could cause irreversible damage to competitors before the probe concludes.
The watchdog has the power to stop the rollout if it can prove that urgent action is needed to prevent significant harm to individuals or businesses in the UK. The CMA can also act if it is in the public interest.
It has not yet decided what action to take, and it would need to meet a set of strict criteria before it could slap Google with an order to freeze its business activities.
Kinga Incze, founder of media consultancy Whitereport, said: 'This initiative is absolutely the right one, and it is one of the first cases in the world where an authority will look at how competition relates to privacy. Google's power can make it very threatening.'
The CMA's probe comes as part of a wider crackdown on the dominance of Google and Facebook in online advertising. The two control around 80 per cent of the UK's £14billion market.
James Rosewell, director of Marketers for an Open Web, said: 'The CMA would not be conducting an investigation if they didn't feel the evidence they have seen showed there was a significant case for action.'
Google said: 'The Privacy Sandbox has been an open initiative since the beginning and we welcome the CMA's involvement as we work to develop new proposals to underpin a healthy, ad-supported web without third-party cookies.'