Web privacy bombshell: Google must reveal details of anyone who's ever watched a YouTube video

The ruling - which has massive privacy implications for millions of internet users - was made as part of the search engine's legal battle with content provider Viacom over allegations of copyright infringement.

Under the ruling, Google, which bought YouTube for $1.65 billion (£820 million) in 2006, must hand over to Viacom its viewing log - which includes users' log-in information and their IP address, the code that identifies their computer.

Google must hand over the viewing details of every single person who has watched a video on its website YouTube

Although the case is being contested in the U.S., legal experts warned last night that the ruling would almost certainly apply to YouTube users worldwide, including those in the UK. The Electronic Frontier Foundation, an internet freedom campaign group, described the judgment as a ' setback' to online privacy rights.

Viacom, which owns MTV and Paramount Pictures, has alleged that YouTube has done 'little or nothing' to stem the flow of copyrighted material on its site.

The company said it had identified more than 150,000 unauthorised clips of its content that had been uploaded.

In one case, the Al Gore documentary An Inconvenient Truth had been viewed 1.5 billion times.

However, Google claims its safeguards already exceed its legal obligations in helping content owners protect their works and says Viacom's lawsuit is a threat to internet freedom.

The search engine said it abides by the rules of the Digital Millennium Copyright Act, which states that as long as sites such as YouTube remove copyright content as soon as it is brought to their attention, they are protected by law from prosecution. Last night, a spokesman for Viacom tried to play down fears that individual YouTube users would be prosecuted.

'Only our legal team has access to the data,' he said. 'We are not going to use it to go after individual users.'

However, the U.S. district court in New York declined Viacom's request that Google be forced to hand over the source code of YouTube - details of how the site works  -  saying it is a trade secret.

Catherine Lacavera, Google's senior litigation counsel, said: 'We will ask Viacom to respect users' privacy and allow us to anonymise the logs before producing them under the court's order.'

Simon Davies, a British privacy expert, said: 'The chickens have come home to roost for Google.

'Their arrogance and refusal to listen to friendly advice has resulted in the privacy of tens of millions being placed under threat.'

A 'dangerous view' of the world

The popularity of Facebook and other social networking websites could be creating a generation of children with a potentially dangerous view of the world, doctors have warned.

They claim the fast pace of the online world means real life may appear boring and unstimulating to youngsters, leaving many with a distorted view of real life.

Some may even place less value on their lives, raising the risk of suicide, psychiatrist Himanshu Tyagi, of the West London Mental Health NHS Trust, told the Royal College of Psychiatrists' annual conference.