International Monetary Fund chief Christine Lagarde has been placed under formal investigation by French magistrates for negligence in a political fraud affair dating from 2008 when she was finance minister, her lawyer said today.

Lawyer Yves Repiquet told Reuters that Ms Lagarde, who this week was questioned by magistrates in Paris for a fourth time under her existing status as a witness in the long-running saga, said the step was unfounded.

"We are appealing it," Mr Repiquet said by telephone, adding that Ms Lagarde, who was due to return to IMF headquarters in Washington later today, had no plans to resign.

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

The inquiry relates to allegations tycoon Bernard Tapie, a supporter of conservative ex-president Nicolas Sarkozy, was improperly awarded €403m in an arbitration to settle a dispute with now defunct, state-owned bank Credit Lyonnais.

The inquiry has already embroiled several of Mr Sarkozy's cabinet members and France Telecom CEO Stephane Richard, who was an aide to Ms Lagarde when she was Mr Sarkozy's finance minister.

In previous rounds of questioning, Ms Lagarde accused Mr Richard of having used her pre-printed signature to sign off on a document facilitating the payment, local media has said.

However Mr Richard has stated that Ms Lagarde was fully briefed on the matter.

Investigators are trying to determine whether Mr Tapie's political connections played a role in the government's decision to resort to arbitration that won him a huge pay-out.

The offence of negligence by a person charged with public responsibility in France carries a maximum penalty of one year's imprisonment and a €15,000 fine.

Ms Lagarde has been managing director of the IMF since July 2011.

The IMF last year said that the global lender's board had discussed possible consequences of the Tapie case and determined that she would be able to lead.