David Shayler, a former MI5 officer, was detained by French police at the request of British authorities, according to a Home Office spokeswoman. He faces charges under the Official Secrets Act and the government is seeking his extradition.
The Home Office denied the arrest was made in response to articles in today's Sunday Times, which reported that Mr Shayler was threatening to publish details of Britain's spying operations on the Internet.
The Home Office spokeswoman said: "French authorities have arrested David Shayler at Her Majesty's Government's request pending a formal request for extradition, which is in preparation."
A Scotland Yard spokesman said: "A 32-year-old man has been arrested this evening by French police at the request of the Metropolitan Police service in connection with an on-going investigation into a possible breach of the Official Secrets Act 1989."
John Wadham, who is director of the civil rights group Liberty and also Mr Shayler's lawyer, said his client would fight the extradition.
"His revelations were in the public interest and these proceedings should be dropped immediately," Mr Wadham said. "David Shayler will fight the extradition because his only fault was to disclose malpractice and abuse within MI5".