A US court has forced Google to disclose the user details of every viewer of its YouTube video site.
The ruling is a result of Viacom's multibillion-dollar lawsuit against Google, which alleges that YouTube has breached copyright rules on its video content.
The court ruling has alarmed privacy campaigners. Google will have to disclose the user log-in names and IP addresses of every viewer who has ever watched content on the YouTube site.
This means even non-members of YouTube could be tracked by Viacom, which says its copyrighted video content has been illegally posted on the YouTube site.
Google says it has no real control over what is posted on YouTube, although it often takes things off the site after complaints.
The mammoth viewing database awarded to Viacom includes the log-in ID of users, user IP addresses, and details of what they have viewed.
To get a positive identification of each user, Viacom would need the details of the ISP used by viewers, but it has not requested these details and has not so far indicated that it is after anyone but Google for allowing the copyrighted content to be posted.
Viacom owns MTV and Paramount Pictures, among other content generators.
The UK Premier League is also seeking class action status with Viacom on the issue. It says YouTube, which was bought by Google in 2006, has been used to watch copyrighted football highlights.
When Viacom initiated its legal action in March 2007, Viacom said it had identified about 160,000 unauthorised clips of its programmes on YouTube, which had been viewed more than 1.5 billion times.
The court also ruled that Google must disclose to Viacom the details of all videos that have been removed from the site for any reason.
Viacom says Google is not doing enough to stop copyrighted content appearing in the first place, although Google has recently introduced some filtering controls.