French prosecutors said on Friday they were examining allegations that Dominique Strauss-Kahn took part in a gang rape in the United States, something the former IMF chief has categorically denied.
Strauss-Kahn, two businessmen and a police chief have already been charged with "aggravated pimping in an organised gang" for allegedly organising a prostitution ring for orgies in France, the United States and elsewhere.
On Friday, a spokesman for the state prosecutor's office in the northern French city of Lille said investigating magistrates had submitted new evidence that could implicate the men in an alleged gang rape in Washington.
"The Lille prosecutor was presented by the magistrates charged with investigating the 'Carlton affair' with a notice to make known, for whatever useful purpose, evidence of gang rape," the spokesman said.
The so-called "Carlton affair" centres around allegations that business leaders and police officials in the northern city of Lille operated a vice ring supplying girls for sex parties, including at Lille's Carlton Hotel.
The latest development sprang from testimony from a Belgian sex worker.
She alleged she was forced to take part in sex acts against her will at a party attended by Strauss-Kahn and his friends in a Washington hotel suite in December 2010.
Another escort present at the party partially confirmed her statement, but neither has sought to press charges.
Strauss-Kahn's lawyers denounced the claims.
Their client "absolutely contests having committed the slightest act of violence of any nature whatsoever," lawyers Henri Leclerc, Frederique Baulieu and Richard Malka said in a statement.
Contesting the testimony from the two escorts, the statement said: "The declarations made by these young women are contradictory.
"Distorted information, manipulation and a desire for a lynching cannot forever be a substitute for reality," the lawyers added.
In Washington, police spokesman Officer Hugh Carew told AFP: "We've had no report of a sexual assault for December 2010 at the W Hotel."
He was unaware of any complaint against Strauss-Kahn in Washington as a whole, he added.
Having received such a notice, the French prosecutor can either throw out the allegation or order a preliminary inquiry to establish if there is enough evidence to expand the original probe to include the newer charge.
The statement from Strauss-Kahn's lawyers also questioned the timing of the release of the information.
Although the information was contained in documents dating back to March 28, it only appeared in French newspapers Friday -- two days before the presidential election run-off between right-wing incumbent Nicolas Sarkozy and Socialist challenger Francois Hollande.
Strauss-Kahn was at one point the frontrunner to lead the Socialist Party against incumbent Nicolas Sarkozy in the vote.
But in May last year, he was arrested in New York after a hotel maid alleged he had subjected her to a brutal sexual assault in his hotel suite.
The criminal case collapsed amid doubts about the alleged victim's testimony, but she is still pursuing a civil suit against him, and on his return to France Strauss-Kahn found himself implicated in other scandals.
Strauss-Kahn has admitted he had a free-wheeling sexual lifestyle when he was managing director of the Washington-based International Monetary Fund.
But he has fiercely denied he was ever violent towards a partner.
The Lille vice ring is alleged to have supplied women for orgies attended by Strauss-Kahn in Paris, Washington and European cities and investigators believe they can prove the former finance minister committed crimes.
Paying for sex is not illegal in France, but profiting from an organised vice ring is. If convicted, Strauss-Kahn could face a lengthy jail term.