French judges have decided not to drop pimping charges against former International Monetary Fund chief Dominique Strauss-Kahn, according to his lawyers.

They had sought to have the case thrown out, and quickly vowed to appeal.

Mr Strauss-Kahn, 63, is fresh off a legal settlement reached in the US last week with Nafissatou Diallo, the New York hotel maid who had accused him of rape in May 2011.

That accusation damaged his once-illustrious political career, and was followed by other allegations of sex-related wrongdoing.

None have led to a conviction. The only case remaining is the investigation into Mr Strauss-Kahn's suspected role in a hotel prostitution ring in the northern French city of Lille.

An appeals court in nearby Douai today maintained preliminary charges filed against Mr Strauss-Kahn in March for "aggravated procurement in an organised gang" - meaning the probe can continue.

He is one of several defendants in the case, which allegedly involves prominent city figures and police.

Mr Strauss-Kahn, a one-time French presidential hopeful, resigned from his IMF job and saw his international reputation collapse and his sexual proclivities aired in public after the claims by Ms Diallo.

Mr Strauss-Kahn's lawyer Henri Leclerc today lashed out at the investigating judges leading the Lille case.

In a statement, he claimed the charges against Mr Strauss-Kahn were not specified, that some evidence was hidden from the defence, that facts were twisted and that the definition of "pimping" was created with no basis in law.

Outside the Douai courthouse, Frederique Baulieu, another defence lawyer, told reporters: "We are certain that Dominique Strauss-Kahn will be cleared of all charges against him."

The case against Mr Strauss-Kahn hinges on whether he knew he was partying with prostitutes and whose money was used to pay them.

His lawyers have said Mr Strauss-Kahn had attended "libertine" gatherings but did not know that some women there were paid.

In France, it is not against the law to pay for sex, but is against the law to solicit or to run a prostitution business.

Two men with ties to Mr Strauss-Kahn are behind bars in the investigation, accused of organising parties involving prostitutes.

Prostitutes questioned in the case said they had sex with Mr Strauss-Kahn during 2010 and 2011 at a luxury hotel in Paris, at a restaurant in the French capital and also in Washington, where he lived while working for the Washington-based IMF, judicial officials have said.

Under French law, preliminary charges mean authorities have reason to believe that a crime was committed but allow more time for investigation.

New York prosecutors dropped their sex assault case against Mr Strauss-Kahn in August last year, saying they had doubts about the hotel maid's trustworthiness.

The settlement last week came as part of a civil case.

A French writer also claimed that Mr Strauss-Kahn tried to rape her in 2003 but French prosecutors threw out her case because it happened too long ago.