French prosecutors dropped a sex assault probe into former IMF chief Dominique Strauss-Kahn on Thursday, saying he admitted trying to kiss the alleged victim but the case was too old to prosecute.

Tristane Banon, a 32-year-old author, claims the politician tried to rape her in 2003, but prosecutors said evidence suggested sexual assault - which has a shorter statute of limitations - rather than attempted rape.

"Mr Strauss-Kahn admitted trying to kiss Miss Banon," a judicial official told reporters.

"He does not admit sexual assault, but that's his opinion. The magistrate for his part said it could be regarded as a sexual assault."

The Paris prosecutor's office said in a statement: "Even if a prosecution for attempted rape could not be launched for lack of sufficient proof, facts that could be qualified as sexual assault were admitted."

Henri Leclerc, one of Strauss-Kahn's lawyers, confirmed the details of the ruling, but insisted: "He admitted no violence of any kind. Mr Strauss-Kahn tried to kiss her. He was refused, and did not insist, allowing her to leave."

Under French law sexual assault is a lesser charge than attempted rape and cannot be prosecuted if a complaint is made more than three years after the alleged incident. The state prosecutor has therefore halted his probe.

Banon had already said she would attempt to bring a private prosecution if the prosecutor's office refused to take the case, in which case an independent investigating magistrate will have to reconsider the evidence.

"He will have to be satisfied with being an unconvicted sex attacker, protected by the statute of limitation from criminal charges, but not from legitimate suspicion about his behaviour towards women," Banon's lawyer said.

The lawyer, David Koubbi, said the decision 'while unsatisfactory, is a first victory for Miss Banon as, after five months of fierce combat, it has been established beyond doubt that her case is not without substance'.

Koubbi argued that the ruling demonstrated that 'the facts that she complained of were not 'imaginary' contrary to Mr Strauss-Kahn's claims'.

Another of Strauss-Kahn's lawyers had an entirely different take on the decision, arguing that her client had been 'completely cleared'.

"When someone is the subject of a complaint and this complaint is shelved without charge, that means there's no grounds for prosecution. That's called being cleared," said Frederique Beaulieu.

Both Banon and Strauss-Kahn, a senior Socialist politician 30 years her senior and a friend of his accuser's family, have been interviewed by police.

She claims he lured her to an unfurnished Paris flat on the pretext of giving her an interview for a book she was researching, then grappled with her 'like a rutting chimpanzee' while attempting to pull off her jeans.

The former IMF director also still faces a civil suit in New York, where another young woman, a hotel chamber maid, claims he tried to rape her in May.

Strauss-Kahn again denied violence but admitted he had had a sexual encounter with the maid during her seven-minute visit to clean his room.

He was charged in a New York court, but prosecutors dropped the case amid doubts over the woman's testimony. She is pursuing a civil case for damages.

The New York scandal effectively torpedoed 62-year-old Strauss-Kahn's plan to challenge President Nicolas Sarkozy in next year's presidential election, forcing him out of a race in which he was the clear favourite.

Since his return to France he has tried to rebuild some of his reputation, apparently with an eye to some kind of return to frontline politics, but the Banon case has continued to dog his steps.

Thursday's decision came on the same day as the release of Banon's latest book, a 128-page novelisation of her experiences, extracts of which will be published in this week's mass circulation Paris Match news magazine.

In 'Le bal des hypocrites' (The Hypocrites' Ball) she does not identify her alleged attacker by name, but refers instead to a 'pig' and a 'baboon man'.