Correction: An earlier version of this story said Dominique Strauss-Kahn had been charged in a prostitution case. The story has been clarified to explain that he faces preliminary charges.

Dominique Strauss-Kahn, former head of the International Monetary Fund and once a leading candidate to become president of France, learned Monday night that he would face a formal investigation on allegations that he helped procure prostitutes for sex parties he attended in Washington, Paris and elsewhere.

The preliminary charges, brought in the northern French city of Lille, marked another sordid chapter in the swift downfall of the 63-old-economist and Socialist Party politician. Until last May he had been considered the main likely candidate — and a probable winner — against President Nicolas Sarkozy in France’s two-round presidential election, scheduled for April 23 and May 6.

The action in Lille was separate from the sexual assault charges brought against Strauss-Kahn in New York last year. Although those charges were later dropped, Strauss-Kahn still faces a civil suit over the accusations by an immigrant maid, Nafissatou Diallo, that he raped her in his Manhattan hotel room as she came in to clean up.

Judge Douglas McKeon is scheduled to hear arguments Wednesday in a Bronx court over Strauss-Kahn’s assertion that Diallo’s suit should be dropped because he had diplomatic immunity through his job as IMF director. Although he had a diplomatic passport at the time, Strauss-Kahn did not invoke that defense during the proceedings over the sexual assault charges.

The preliminary charges in Lille — procurement as part of a group of acquaintances based in the city — meant the magistrates there concluded that Strauss-Kahn’s conduct, along with that of the others, should be investigated further with an eye to putting him on trial. They did not mean he was accused of employing or profiting from the work of prostitutes. He was later freed on bail.

Under French law, prostitution is tolerated but procurement is a crime punishable by up to 20 years of imprisonment, meaning that, in theory, Strauss-Kahn could risk going to jail. The magistrates did not pursue the other allegations under investigation: that Strauss-Kahn participated in corruption because his partners in the sex parties put prostitutes’ bills and other charges on their company expense accounts.

Strauss-Kahn offered no comment as he left the courthouse after about eight hours of questioning. But one of his attorneys, Richard Malka, who announced the magistrates’ decision, said his client maintained that he was innocent of any crimes, as he has throughout the investigation, because he thought the women at the sex parties were just out for a good time.

“He declares with the greatest firmness that he is not guilty of any of these things and that he was never in the least aware that the women he met were prostitutes,” Malka told reporters outside the courthouse in Lille.

The Lille prosecutor’s office announced that Strauss-Kahn was put under judicial control, meaning that he must remain at the disposal of police and investigating magistrates, who can also limit his travel.