The Shayler case, which has already re-opened the debate on the Act in Britain, may also become a cause celebre in French and European, law.
At his 90-minute extradition hearing in Paris yesterday, both the French state and Mr Shayler's lawyers argued that the decision on whether to return him to Britain could have an important impact on future, judicial relations between EU countries.
In effect, the Chambre d'Accusations must decide in the the next four weeks whether Mr Shayler committed a "political" crime and whether the concept of "political crime" still exists between EU nations.
Mr Shayler, in a brief statement to the court, said: "The only thing I have ever done ... is to criticise the (British) state."
He said his motive in leaking MI5 secrets to the press was to expose the shortcomings of the British security services. He denied an accusation from the French public prosecutor that he had sold the information simply to make money.
He had accepted pounds 20,000 from the Mail on Sunday, he said, because he knew he would have to flee abroad and would need cash to survive. He had since offered to return the money.
Mr Shayler, 32, a junior officer in MI5 for four years, was arrested in Paris in August at the request of the British police. He is charged with a breach of the Official Secrets' Act after leaking 30 top secret documents which were the basis of a Mail on Sunday article on bungling, incompetence and persistent drunkenness in MI5.
Mr Shayler repeated his allegation yesterday that he was arrested to prevent him leaking further, embarrassing information about a failed British plot to assassinate the Libyan leader, Colonel Gaddafi - a failure which led, he said, to the deaths of "innocent civilians".
It was Mr Shayler's first public appearance since his arrest. He had shaved off his goatee beard and looked pale and thinner after nearly three months in the Sante prison in Paris.
France does not extradite people for "political crimes", but a 1996 European convention committed EU countries to abolish the concept of "political" crime. France's constitutional council ruled that this treaty infringed a "fundamental principle" of French law and could not be ratified.
Mr Shayler's lawyer, Maitre William Bourdon, argued that the British police had admitted that no vital state or defence secret had been threatened. By negotiating a possible deal or amnesty with Shayler, in effect they admitted that his case was a political, not a criminal one.
Tony Blair is expected to appoint a Whitehall "spywatcher" to reinforce its checks on Britain's secret and intelligence services - MI5, MI6 and GCHQ - following warnings by the cross-party Intelligence and Security committee that their powers needed strengthening, writes Our Chief Political Correspondent.Reuse content