Lagarde is summoned to appear before a special court devoted to cases of ministerial misconduct on 23 May to answer accusations that she favoured Bernard Tapie in a financial dispute that resulted in the businessman and politician being paid 400 million euros.
Tapie, who had been a minister under Socialist president François Mitterrand in 1992-93, went on to support Sarkozy in the 2007 and 2012 presidential elections and prosecutors with the Court of Justice of the Republic (CJR)suspect that the settlement was a payoff for his support.
They have described Lagarde’s handling of the case as "questionable" and suggested she was partly responsible for "numerous anomalies and irregularities" that could lead to charges for complicity in fraud and misappropriation of public funds.
Her lawyer, Yves Repiquet, described the summons as a “non-event”, saying that it “has been expected for several months”.
Lagarde denies the accusations and has refued to resign from the IMF post, which she took after the resignation of another French former finance minister, Dominque Strauss-Kahn.
The investigation centres on Lagarde’s decision to go to private arbitration in a dispute between Tapie and the partly state-owned Crédit Lyonnais bank over the 1993 sale of sports group Adidas.
The Mediapart website claims to have a letter from experts that undermines her defence, advising her not to pay any money to Tapie while possibly saving him from the bankruptcy with which he was then threatened.