Former International Monetary Fund head Dominique Strauss-Kahn is due to find out whether he will stand trial on pimping charges.
The charges relate to sex parties held in a luxury hotel in the northern French city of Lille.
Mr Strauss-Kahn's lawyer has asked for the case to be dropped, saying his client did not know that some of the women at the parties were prostitutes.
It is the only outstanding case Mr Strauss-Kahn faces in France.
Mr Strauss-Kahn, 63, was widely expected to become the Socialist presidential candidate before he was arrested in May 2011 in New York, accused of trying to rape a hotel maid.
US prosecutors later dropped criminal charges, though the alleged victim, Nafissatou Diallo, is pursuing a civil lawsuit alleging sexual assault.
The Lille case has become know as the Carlton affair, after the name of the hotel in which the alleged orgies took place.
Consorting with prostitutes is not against the law in France, and Mr Strauss-Kahn has acknowledged that he was at some of the parties with the women.
But Mr Strauss-Kahn's legal team says he had no idea they were prostitutes, and that there is no evidence to support a formal charge of pimping.
"We are requesting that this investigation be annulled on account of the fact that there are insufficient grounds to support it," said defence lawyer Richard Malka.
Mr Strauss-Kahn, who is reportedly taking steps to reinvent himself as a highly paid consultant and conference speaker, says the authorities are trying to "criminalise lust".
Other cases against him have already been dropped.
Last month, French prosecutors ended an investigation into allegations of "gang rape" at a hotel in Washington after the woman who made the claim retracted her evidence.
Magistrates also dropped a sexual assault case brought by French author Tristane Banon on the grounds that the alleged 2003 incident had taken place too long ago.