International Monetary Fund head Christine Lagarde was placed Wednesday under official investigation for negligence in a French corruption probe that dates back to her days as France's finance minister.

Calling the allegations "without basis" Lagarde said she would return to her work in Washington after a fourth round of questioning by French magistrates.

The IMF chief and a former chief of staff are being probed about a payment to businessman Bernard Tapie worth more than $500 million while she was finance minister about a half-decade ago.

Tapie was paid the sum in arbitration over a dispute between French bank Credit Lyonnais during the bungled sales of sportswear firm Adidas in the early 1990s.

Critics say the payment was part of a boondoggle and a symptom of the too cozy relationship between business and the French government at the time.

The payment was made much later, when Lagarde was in power.

"After three years of proceedings, dozens of hours of questioning, the court found from the evidence that I committed no offence, and the only allegation is that I was not sufficiently vigilant," she said in her statement.

Under French law, the official investigation is equivalent to preliminary charges, meaning there is reason to suspect an infraction. Investigating judges can later drop a case or issue formal charges and send it to trial.