Former International Monetary Fund boss Dominique Strauss-Kahn has been charged over alleged involvement in a prostitution ring.
He was charged after being questioned by magistrates in the northern city of Lille.
Mr Strauss-Kahn has admitted attending parties where the authorities believe prostitutes were provided by a gang.
But he denies knowing that the women were paid prostitutes.
Last May he resigned from the IMF after being accused of attempting to rape a hotel maid in New York. The charges were later dropped.
The maid is now bringing a civil case against Mr Strauss-Kahn, which is due to start in New York on Wednesday.
He has always denied any wrongdoing.
The three investigating judges in Lille are looking into what has become known in France as the "Carlton affair", named after the hotel where several orgies are said to have been held.
They are examining allegations that business associates of Mr Strauss-Kahn were involved in a prostitution ring, and misusing corporate funds.
They have placed Mr Strauss-Kahn under formal investigation.
One of his lawyers, Richard Malka, said: "He firmly declares that he is not guilty of these acts and of never having the least inkling that the women he met could have been prostitutes."
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.
Last month he was held in police custody for 48 hours at the start of his formal questioning.