Ms. Lagarde said in a statement through her lawyer that she was being investigated for “simple negligence,” by the Court of Justice of the Republic, the judicial body that is charged with investigating the conduct of high government officials.
Ms. Lagarde, who has denied wrongdoing from the start, said in a statement that the decision by a judicial committee to place her under formal investigation was “totally unfounded” and that she would be returning to work in Washington on Wednesday afternoon.
She made the announcement a day after she was questioned for a fourth time in the investigation of her role in 2008 during an arbitration proceeding between the government and Bernard Tapie, a onetime cabinet minister and the former owner of the Adidas sportswear empire. Prosecutors had assigned Ms. Lagarde the status of “assisted witness” in the case. Placing her under formal investigation signals that prosecutors believe they have evidence of wrongdoing, but the charge of negligence hardly suggests high crimes.
Negligence by a government official that makes possible the misappropriation or embezzlement of public funds is punishable with a maximum fine of 15,000 euros, or $19,800, and up to one year in prison.
“After three years of investigation and dozens of hours of interrogation, the committee concluded that I had not been guilty of any infraction,” Ms. Lagarde said in the statement, “so it was reduced to claiming that I had been insufficiently vigilant during the arbitration.”
Under the French law, a formal investigation suggests prosecutors believe they have enough of a case that they may ultimately bring criminal charges and trial. The reality is that many such investigations drag on for years with no charges being filed before being dropped.
Since its creation in 1944, the top I.M.F. official has been a European, with the job alternating among Western European nations. Ms. Lagarde took over as managing director of the I.M.F. in 2011 after her French predecessor, Dominique Strauss-Kahn, left in disgrace amid charges — later dropped — that he had sexually assaulted a woman in a New York hotel room.
Ms. Lagarde had served as Mr. Sarkozy’s finance minister from 2007 until leaving for the I.M.F.
Mr. Tapie, who had been in a long dispute with a state-owned bank, Credit Lyonnais, walked away from the arbitration with more than €400 million.
A career Socialist, Mr. Tapie in 2007 suddenly backed Mr. Sarkozy’s center-right Union for a Popular Movement party. The amount of the award came as a shock to many, and the investigation began after critics accused Mr. Sarkozy’s government of having carried out a sham arbitration.
Stéphane Richard, Ms. Lagarde’s former chief of staff and currently the chief executive of Orange, the French telecommunications giant, has already been placed under formal investigation on “suspicion of organized fraud.”
Prosecutors in Paris could not immediately be reached for comment.
In a statement, the I.M.F. communications director, Gerry Rice, said Ms. Lagarde was “now on her way back to Washington and will, of course, brief the board as soon as possible. Until then, we have no further comment.”
Ms. Lagarde’s lawyer did not immediately return a request for comment, but Ms. Lagarde told Agence France-Presse that she had no intention of stepping down from her post at the monetary fund.
