Twitter said it was prepared to hand over information identifying tens of thousands of people who have used the social-networking website to break privacy injunctions.
A senior executive from Twitter yesterday admitted for the first time that the website would turn over information to authorities in Britain if it was "legally required" to do so.
Experts had previously assumed that people who breached gagging orders on Twitter were protected from legal reprisals because the website is outside the jurisdiction of British courts.
The admission came after Dominic Grieve, Britain's Attorney General, warned earlier this week that people who breached injunctions online were in for a "rude shock".
He said: "It is quite clear, and has been clear for some time in a number of different spheres, that the enforceability of court orders and injunctions when the internet exists into which information can be rapidly posted, that presents a challenge.
"But that doesn't necessarily mean that the right course of action is to abandon any attempt at preventing people from putting out information which may in some circumstances be enormously damaging to vulnerable people or indeed, in some cases, be the peddling of lies."
Ryan Giggs, the Premiership footballer, last week started legal proceedings against Twitter and "persons unknown" after more than 70,000 users revealed that he had obtained an injunction to hide an extra-marital affair.
On Monday John Hemming, the back-bench Liberal Democrat MP, used parliamentary privilege to identify Giggs in Parliament.
The admission by Twitter, however, could encourage legal action by a number of celebrities who have been named on the website as having obtained injunctions to hide alleged affairs.
Tony Wang, Twitter’s head of European operations, said yesterday the website would notify users in advance so they could fight the application in the court before Twitter handed over the information.
He said: "Platforms have a responsibility, not to defend that user but to protect that user's right to defend him or herself.
"If we're legally required to turn over user information, to the extent that we can, we want to notify the user involved, let them know and let them exercise their rights under their own jurisdiction.
"That's not to say that they will ultimately prevail, that's not to say that law enforcement doesn't get the information they need, but what it does do is take that process into the court of law and let it play out there."
But Mark Stephens, a media lawyer at Finer, Stephens Innocent, said it would still be impossible to track anonymous users if they used fake email addresses and set up Twitter accounts in internet cafes.
He said: "What will happen is that people will use anonymised accounts and internet cafes to break injunctions. There is nothing they [the authorities] can do about that, they will have no idea who they are. It is like stopping rain, as long as there is anonymity they will be protected."
When Mr Hemming named Giggs earlier this week he said: “With about 75,000 people having named Ryan Giggs on Twitter, it is obviously impracticable to imprison them all.”
Earlier this month the High Court gave the green light for Louis Bacon, a billionaire US hedge fund manager to force Wikipedia, the online encyclopaedia, to disclose the identities of online commentators who allegedly defamed him.