IMF chief Christine Lagarde today said she is under official investigation for negligence in a French corruption probe that dates back to her days as finance minister.

In a statement after a fourth round of questioning before magistrates, Lagarde said she was returning to her work in Washington later in the day and said the decision was "without basis."

She and her former chief of staff face questions about their role in a 400 million-euro (USD 531 million) payment to a businessman.

The payment was made to Bernard Tapie in a dispute with state-owned bank Credit Lyonnais over the botched sale of sportswear company Adidas.

Critics have said the deal was too generous, and was symptomatic of the cozy relationship between money and power in France.