Doubts cast on the Dominique Strauss-Kahn sex assault case raised hopes among France's opposition on Friday that he may return to help them fight Nicolas Sarkozy for the presidency next year.

Strauss Kahn's political career was written off along with his role as head of the International Monetary Fund (IMF) when he was arrested in New York on May 14, charged with sexually assaulting and trying to rape a hotel maid.

He had previously polled as the candidate most likely to beat Sarkozy.

But when the New York Times reported on Thursday that the case against Strauss-Kahn was collapsing due to doubts over the victim's credibility, his French Socialist allies immediately saw fresh political hope.

"It's a thunderbolt - but in the opposite direction this time," said Socialist former prime minister Lionel Jospin, referring to the shock when the news of Strauss-Kahn's arrest broke on May 14.

Strauss-Kahn's departure from the presidential race prompted the Socialist Party's leader Martine Aubry to step up and declare her candidacy this week, running for the nomination against former party leader Francois Hollande.

The deadline for applying for the party nomination is July 13. A close party ally of Strauss-Kahn, Michele Sabban, called for the primary process to be suspended so he has time to run.

Party spokesman Benoit Hamon said such a suspension was not currently under discussion.

Others were too cautious to suggest Strauss-Kahn would try to run for president after the scandal, but agreed he could have a huge impact on his party's morale if he were cleared of the accusations and returned to politics.

"His presence alongside us would be decisive for our success in the presidential election," said Jack Lang, a leading Socialist former minister, on BFMTV.

French politicians had insisted on Strauss-Kahn's right to be presumed innocent until US justice had run its course.

The third Socialist candidate in the race, former presidential contender Segolene Royal, highlighted that the final judgment is still pending.

"It is urgent that the truth come out," she told a televised news conference," but played down the prospect of a triumphant return by Strauss-Kahn.

"In human terms, that's not likely to be one of his priorities," she said.

"The trauma suffered by him and those close to him is after all quite dreadful."

Aubry said on Friday the New York Times report gave her 'immense joy' and hoped it would allow him to 'end his nightmare', but did not comment on how it would affect the party's primary to choose a presidential candidate in October.

Citing law enforcement sources, the New York Times said prosecutors did not believe much of the story from the maid and suspect she has repeatedly lied to them. Strauss-Kahn denies all the charges against him.

The district attorney's office unexpectedly said that Strauss-Kahn would make a court appearance on Friday. The newspaper said the judge may ease the bail conditions which have the suspect under house arrest in a Manhattan apartment.

"The session this afternoon in the Manhattan court will be an essential judicial step," Hollande, currently polling as the favourite to win the Socialist nomination to run for president, told AFP.

"I hope it will knock down all the charges which have weighed on him so painfully and cruelly."

Sarkozy has not publicly commented on the scandal, which raised the faint prospect of him recovering from his low approval ratings to win re-election against a shaken and shamed opposition, despite their lead in the polls.

"I think Sarkozy and his friends had a very rude awakening this morning," said Socialist MP Claude Bartolone on Friday.