Google has been ordered to expose the viewing habits of every user who has watched YouTube, the internet's largest video site, it emerged last night.

The order from a district court judge in New York has raised concerns among users and privacy campaigners that the online video viewing habits of hundreds of millions of people worldwide could be revealed.

The ruling came as part of Google's legal battle with Viacom, the parent company of Paramount Pictures, Dreamworks and MTV over allegations of copyright infringement.

Details that would be made available to Viacom include log-in details of YouTube subscribers or viewers, the IP address of the PC or laptop used and video clip details.

But Google and Viacom, have argued that such data cannot be used to unmask the identities of individual users with certainty. They said they were working together to ensure data is further "anonymised".

Viacom has assured web surfers the company will not use the user logs it gets from Google to prosecute individual YouTube users.