A former MI5 officer, Mr David Shayler, has been remanded to a Paris prison ahead of proceedings to bring him back to Britain, French authorities said last night.
The maverick security service agent was arrested in the French capital on Saturday night amid rumours that he was about to publish details of MI5 operations. The British government is to seek his extradition.
Lawyers for Mr Shayler (32) vowed yesterday to fight "absolutely" his return to Britain to face prosecution under the Official Secrets Act for allegedly disclosing sensitive information from top-secret files.
The French public prosecutor in charge of the case confirmed that Mr Shayler had been brought before her after his detention by three French plain-clothes officers in a bar near his hotel.
Ms Isabelle Chauvin, deputy public prosecutor at the Parquet de Paris, the French public prosecutions office, said: "Mr David Shayler has been remanded in custody ahead of extradition proceedings.
"He is the subject of a process under French law in which a request from the British authorities for extradition will be considered and then a decision made. In the meantime he will remain in custody."
Mr Shayler went to Paris from the remote French farmhouse he shares with his girlfriend and fellow former MI5 worker, Ms Annie Machon, amid rumours that he was about to release damaging information on the Internet. He was being held at the main jail in Paris, the Prison de la Sante.
The Home Office confirmed it now has 40 days to file an extradition request with the French authorities.
A Home Office spokeswoman said: "The French authorities arrested David Shayler at Her Majesty's Government's request pending a formal request for extradition. That request is now in preparation."
Mr John Wadham, director of civil rights group Liberty, which has taken up the Shayler case, said: "It is our intention to fight absolutely moves to bring my client back to Britain. The attempts to bring him back to Britain are the result of a mess which is of the British authorities' own making."
The former MI5 man fled Britain after a series of newspaper articles disclosing that MI5 had kept files on Trade and Industry Secretary Mr Peter Mandelson and Home Secretary Mr Jack Straw, when they were student activists.
He also alleged that intelligence about a planned bombing of the Israeli embassy in London was not passed on.
A High Court injunction prevents detailed publication of the allegations in the UK.
Mr Wadham, who is also representing a former MI6 agent, Mr Richard Tomlinson (35), said earlier that no jury would convict Mr Shayler because his revelations had been made in the public interest.
An angry Ms Machon accused the Attorney General, Mr John Morris, of authorising Mr Shayler's arrest while continuing with negotiations to bring the MI5 officer back to Britain.
Mr Tomlinson, who met Mr Shayler for the first time while in Paris, had been arrested by the French authorities at his Paris hotel on Friday after fears that he was to publish a book in Australia detailing MI6 activities.