A 32-year-old former MI5 officer who revealed secrets about the intelligence service published in a series of newspaper articles last year and then fled across the Channel into self-imposed exile.
What has he revealed in the past?
He said that MI5 kept files on Jack Straw, the Home Secretary, and Peter Mandelson, the Secretary of State for Trade and Industry, when they were student activists.
He also claimed that intelligence about a planned bomb attack on the Israeli embassy in London was not passed on. A High Court injunction prevents detailed publication of the allegations in the UK.
Why was Mr Shayler arrested on Saturday?
Though the Home Office will not comment on the reason there is widespread speculation it was linked to his plan to publish M15 secrets on the Internet.
It has been claimed that among these secrets were details of a British plan to assassinate the Libyan leader, Colonel Muammar Gaddafi, and allegations of a breakdown in communication between Britain's intelligence services which allowed the IRA to carry out a number of "spectaculars" - major bomb attacks.
What will happen to Mr Shayler now?
In the first instance, Mr Shayler's case will go before the Chambre d'Accusation, which must decide whether there is a prima facie case for his extradition to Britain. The British government has 40 days to present its evidence and the Chambre must then make its decision within 45 days.
Under French extradition procedure - although not formally under French law - the Prime Minister, Lionel Jospin, decides whether or not to follow the Chambre's advice. Once he has pronounced, the defence lawyers can, in theory, lodge appeals right up to the Conseil d'Etat - the final constitutional appeal court.
Under a strict interpretation of French law, Mr Shayler could be returned to Britain while such appeals were being prepared and heard. Under French legal tradition, however, the suspect is always held in France until the appeals process is exhausted.
Who is Richard Tomlinson and how is he linked?
Richard Tomlinson, a former SAS member, has just finished serving a 12- month sentence for breaching the Official Secrets Act when he sent a synopsis on a book to an Australian publisher.
He travelled to France last week without a passport and was arrested on Saturday night. Mr Tomlinson, who was released without charge, has said he has no plans to pursue his book deal. He dismissed speculation that he was in Paris to co-operate on a book with Mr Shayler.
Andrew Buncombe and Linus GregoriadisReuse content