Although there is no specific appeal process for extradition cases, the French Justice ministry can seek to have the judgment set aside by the Cour de Cassation, the highest appeal court, if it believes that a wrong and dangerous precedent has been set in French law.
Sources close to the ministry said yesterday that it was "very likely" to take this course, although no decision had yet been made.
The written judgment in the Shayler case, details of which have been seen by The Independent, is unexpectedly sweeping.
It declares, in effect, that anyone revealing the defence secrets of another nation - even a French ally - has committed a political act and cannot be extradited from France.
The French government regards this as a legally flawed and politically nightmarish judgment. It creates a precedent that would allow politically motivated whistleblowers, or straightforward traitors, from friendly nations to seek indefinite asylum in France.
Cases before the Cour de Cassation are notoriously slow; it could take eight months or more before the French government's protest is heard and judged.
In the meantime, Mr Shayler, 32, cannot be touched by the British police and courts, so long as he remains on French soil.
It is understood that Mr Shayler has sold his story to a British Sunday newspaper. He, his girlfriend and two brothers are believed to be staying, at the newspaper's expense, at the Hotel Raphael on the Avenue Kleber, close to the Arc de Triomphe, one of the three or four most expensive and exclusive hotels in Paris (up to pounds 700 a night for a suite).
Mr Shayler, a junior MI5 official for four years, is wanted in Britain for breach of the Official Secrets Act after leaking 30 top-secret documents and details of alleged incompetence, bungling and drunkenness in the security services, to The Mail on Sunday. He was arrested in Paris in August, at Britain's request.
On Wednesday the Chambre d'Accusation of the Paris appeal court ruled that he could not be extradited to Britain.
Shayler's lawyers said that the court had accepted their arguments that Shayler had committed a "political" offence, since he had sought, in the public interest, to draw attention to the shortcomings of an important state agency.
Under French law, extradition for "political offences" is not permitted.
However, the details of the judgment leaked yesterday show that the court took its decision on unusually sweeping grounds. It barely seems to have considered the details of Mr Shayler's actions and motives. The operative paragraphs of the judgment read: "Under French law, any offence which attacks the organisation and conduct of the public authorities is considered a political offence.
"Article 413-10 of the [French] penal code forbids the revelation of national defence secrets ... (which is considered) an attack on the fundamental interests of the nation. Offences described under article 413-10 of the penal code [ie the leaking of secrets] are by their very nature, political offences.
"It follows that, applying article 3 of the European extradition convention [which forbids extradition for political offences], this [ie Shayler's] extradition must be refused."
In other words, the leaking of defence or other state secrets is a political offence. Therefore, nobody who leaks the secrets of another nation can be extradited from France.
The French government believes that this sets a dangerous and embarrassing precedent, which could make France, in effect, a haven for whistleblowers from Britain and other friendly nations.
It also runs counter to moves within the European Union to abolish, in effect, the concept of political crime between EU states and make extradition more or less automatic. A new European extradition convention, along these lines, was agreed in 1996 but has not been ratified by either France or Britain.Reuse content