Twitter has been forced to hand over the personal details of a British user in a libel battle that could have huge implications for free speech on the web.

The social network has passed the name, email address and telephone number of a south Tyneside councillor accused of libelling the local authority via a series of anonymous Twitter accounts. South Tyneside council took the legal fight to the superior court of California, which ordered Twitter, based in San Francisco, to hand over the user's private details.

It is believed to be the first time Twitter has bowed to legal pressure to identify anonymous users and comes amid a huge row over privacy and free speech online.

Ryan Giggs, the Manchester United footballer named as being the plaintiff in a gagging order preventing reporting of an alleged affair with a reality TV model, is separately attempting to unmask Twitter users accused of revealing details of the privacy injunction.

However, Giggs brought the lawsuit at the high court in London and the move to use California courts is likely to be seen as a landmark moment in the internet privacy battle.

Ahmed Khan, the south Tyneside councillor accused of being the author of the pseudonymous Twitter accounts, described the council's move as "Orwellian". Khan received an email from Twitter earlier this month informing him that the site had handed over his personal information. He denies being the author of the allegedly defamatory material.

"It is like something out of 1984," Khan told the Guardian. "If a council can take this kind of action against one of its own councillors simply because they don't like what I say, what hope is there for freedom of speech or privacy?"

Khan said the information Twitter handed over was "just a great long list of numbers". The subpoena ordered Twitter to hand over 30 pieces of information relating to several Twitter accounts, including @fatcouncillor and @ahmedkhan01.

"I don't fully understand it but it all relates to my Twitter account and it not only breaches my human rights, but it potentially breaches the human rights of anyone who has ever sent me a message on Twitter.

"A number of whistleblowers have sent me private messages, exposing any wrongdoing in the council, and the authority knows this."

He added: "I was never even told they were taking this case to court in California. The first I heard was when Twitter contacted me. I had just 14 days to defend the case and I was expected to fly 6,000 miles and hire my own lawyer – all at my expense.

"Even if they unmask this blogger, what does the council hope to achieve? The person or persons concerned is simply likely to declare bankruptcy and the council won't recover any money it has spent."

A spokesman for south Tyneside council said the legal action was brought by the authority's previous chief executive, but has "continued with the full support" of the current head.

He added: "The council has a duty of care to protect its employees and as this blog contains damaging claims about council officers, legal action is being taken to identify those responsible."

Twitter had not returned a request to comment at time of publication.