(Reuters) -IMF chief Christine Lagarde has been put under formal investigation by French magistrates for alleged negligence in a political fraud affair dating from 2008 when she was finance minister.

Lagarde was questioned by magistrates in Paris this week for a fourth time under her existing status as a witness in the long-running saga over allegations that tycoon Bernard Tapie won a large arbitration payout due to his political connections.

"After three years of procedure, the sole surviving allegation is that through inadvertence or inattention I may have failed to intervene to block the arbitration that brought to an end the longstanding Tapie litigation," she said in a statement on Wednesday. "I have instructed my lawyer to appeal this decision, which is without merit."

Under French law, magistrates place a person under formal investigation when they believe there are indications of wrongdoing, but that does not always lead to a trial.

Lagarde's lawyer, Yves Repiquet, told Reuters he would personally appeal the magistrates' decision.