Orange Chief Executive Stephane Richard was at the time chief of staff to Lagarde, then French finance minister. Richard has yet to respond to the subpoena, sources close to the case said on Thursday.

Richard, who is himself under investigation in the case, considers it is not legally possible for him to testify in a case in which he is also under investigation and could ultimately face trial, a source said.

A French court can permit the absence of a witness if they provide a valid reason.

Lagarde is to stand trial from December 12-20 before the Cour de Justice de la Republique, a special court that tries ministers for alleged crimes in office.

She is accused of negligence leading to the misuse of public funds, by improperly approving a decision to allow out-of-court arbitration in a dispute with Tapie, a supporter of conservative former president Nicolas Sarkozy.

A Paris appeals court has ordered Tapie to reimburse the state, but the businessman has lodged an appeal, which is pending.

Tapie sued the state for compensation after selling his stake in sports company Adidas to state-controlled Credit Lyonnais in 1993. He claimed the bank had defrauded him after it later resold his stake for a much higher sum.

(Reporting by Chine Labbé; writing by Leigh Thomas; editing by Andrew Roche)