IMF chief Christine Lagarde is being investigated in a corruption case dating from her time as French finance minister.

But Lagarde, who French authorities have accused of "negligence", said she will not resign from her top role at the global economics body.

"I return back to Washington where I will indeed brief my board," she told the AFP news agency after revealing that she had been placed under formal investigation and denied any wrongdoing.

The case involves a near €400m (£318m, $527m) payout to French businessman Bernard Tapie over his claim to have been conned by the formerly state-owned bank Credit Lyonnais, which had assisted in his sale of sportswear firm Adidas in the early 1990s.

Credit Lyonnais had sold Adidas for a larger sum than he had been told and so Tapie had not received all of the money. A court ruled Tapie had been defrauded.

Lagarde gave the green light to the payout in 2008 when she decided not to appeal against it.

Tapie has been accused of conspiring with the then French government, which was under the leadership of president Nicolas Sarkozy, to secure the large sum and end his battle with Credit Lyonnais. He had backed Sarkozy's presidential bid in 2007.