The personal viewing habits and online identities of millions of YouTube users are to be handed over to an American media giant after a court rejected arguments that such a move amounted to a massive invasion of internet privacy.

Under the terms of a US court judgment, Google, which owns YouTube, must now surrender the details of video-watching histories, IP addresses and usernames, to Viacom, which wants to use the data to prove that the site is hosting thousands of television and other media clips in breach of strict copyright laws.

Last night, Google said it intended to comply with the court order and confirmed the ruling would affect "our global log". At this stage, the company had no plans to appeal against the ruling, a Google spokesman said.

Privacy groups described the ruling as a dangerous precedent that could lead to worldwide breaches of internet privacy.

The Electronic Frontier Foundation, an internet rights group, said the ruling will allow Viacom to see what everyone is watching on YouTube. "We urge Viacom to back off this overbroad request and Google to take all steps necessary to challenge this order and protect the rights of its users," the group said.

Viacom, which owns several US television networks including MTV and Nickelodeon, alleged in a $1bn (£50m) US lawsuit launched in March 2007 that almost 160,000 unauthorised clips of its programmes are available on YouTube. Those clips have been viewed more than 1.5 billion times, the company claims.

Its lawyers argued it needed access to the information on user viewing habits to prove that copyright-infringing material is more popular than user-generated videos on YouTube, which would strengthen its case against Google.

In its judgment, the US District Court for the southern district of New York agreed with Viacom and ordered Google to turn over the information. Google argued that user data should not be handed over because of privacy concerns, but Judge Louis Stanton dismissed those concerns as "speculative". Viacom also asked for the underlying code that Google and YouTube use to search for keywords and video in order to demonstrate what Google could be doing to block infringing videos. They also wanted access to Google's advertising database scheme in the hopes of proving that infringing videos are driving advertising revenue.

But the court rejected all of those requests, arguing that the code and advertising data was too valuable to Google.

Google hopes to limit the damage of the ruling by seeking assurances from Viacom that it will respect users' privacy.

Google's senior litigation counsel, Catherine Lacavera said yesterday: "We are pleased the court put some limits on discovery, including refusing to allow Viacom to access users' private videos and our search technology. We are disappointed the court granted Viacom's overreaching demand for viewing history. We will ask Viacom to respect users' privacy and allow us to anonymise the logs before producing them under the court's order."

Asked what the company would do if Viacom declined to co-operate with its request, a Google spokesman said: "All we can say right now is that we ask Viacom to respect users' privacy and allow us to anonymise the logs before producing them under the court's order. We cannot comment further."

Viacom's claims

160,000 The number of its unauthorised programmes that the US media company alleges are available to watch on YouTube.

1,500,000,000 The number of times that Viacom claims those clips have been viewed.

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