TERRIFIED about details of his private life, and especially the existence of his illegitimate daughter Mazarine becoming public knowledge, president Francois Mitterrand ordered illegal phone tapping and spying on private citizens on a vast scale.

It has been known for a while that the Elysee Palace, during the socialist leader's 14-year stay there, was indiscreet to say the least. Only last week, however, did legal inquiries reveal the full extent of the snooping and the president's obsessive, day-to-day involvement with it.

The latest revelations in a scandal compared yesterday to president Nixon's Watergate were by Christian Prouteau, former head of the Elysee's anti-terrorist squad. Appointed a Prefect in 1985, Prouteau was described by Mitterrand as ''an extraordinary man ... the prototype of what our army can produce''.

That gratitude was again expressed in February 1993, when the president, already desperately ill, awarded his extraordinary man the Legion d'Honneur, France's highest honour.

Prouteau was questioned last week in connection with charges of invasion of privacy brought by journalists and lawyers who were spied on during his years in the Elysee, especially between 1984-86. At the heart of the latest inquiries are a range of documents discovered in February after an anonymous tip-off in a garage rented by Prouteau. Next to spare parts of a Renault 25, an instructing magistrate, accompanied by DST internal intelligence service officers, found an extraordinary stock of ultra-confidential material piled up in 40 cardboard boxes. One file was devoted to telephone tapping.

Prouteau, it transpired, had found no better hiding place for state secrets which he had saved from the shredder when President Mitterrand left office in 1995.

Most of the files were removed by DST officers on the grounds of their security status. Those retrieved by the magistrate, however, contained reports addressed by Prouteau to President Mitterrand, on which the latter had written annotations and comments - proof, it seems, of an earlier claim by Prouteau that all the investigations carried out by himself and his men had been ordered.

The president's main obsession was his daughter Mazarine, born in 1974 to Anne Pingeot who is now senior curator in Paris's Musee d'Orsay. When the writer Jean-Edern Hallier threatened to review the existence of Mazarine in 1984, the Elysee had his private telephone line tapped along with that of his favourite restaurant.

According to the garage documents, the president demanded daily accounts of Hallier's activities and contacts. Informed that the writer was due to appear on a live television programme in March 1984, the Elysee had the broadcast scrapped.

But Prouteau did even stranger things. He drew up a report proposing to weaken Mitterrand's political opponents by tapping their phone lines, investigating their private lives, and disrupting public meetings at which they spoke.

The garage files also include details about services rendered to political friends and informers - parking fines were obligingly cancelled, and proposals made for Legion d'Honneur awards.

The president's spy also had business connections. He set up a meeting between the managing director of IBM France and Andre Rousselet, the president's former head of cabinet. IBM won the contract for the computer system in Prouteau's offices.

Yesterday Prime Minister Alain Juppe ruled that the secret documents taken from Prouteau's garage by the DST had to remain confidential. Those in the possession of the instructing magistrate are doubtless enough to see Prouteau sentenced.

However President Mitterrand, had he lived, would not have been bothered. French presidents may only be prosecuted for high treason, and that is even more difficult to prove.