French prosecutors placed far-right leader Marine Le Pen under formal investigation on Friday as part of an inquiry into the alleged misuse of EU funds to pay parliamentary assistants.
The Front National leader, who is being investigated for breach of trust, has previously denied any wrongdoing in a case that she has said is politically motivated.
In a statement, her lawyer said she would launch an appeal against the investigation.
The case relates to an amount of about €5 million.
It was announced Ms Le Pen had been placed under formal investigation after she appeared before judges on Friday.
She had been summoned previously, but refused to go during her campaign bid for the French presidency.
The European Parliament believes the EU money went to Front National employees working for the party in France, rather than those working for the party’s MEPs in Brussels.
The formal investigation comes less than two weeks after Ms Le Pen won a seat in the French National Assembly. She is no longer a member of the European Parliament.
Ms Le Pen also faces an investigation into the financing of past election campaigns and an inquiry into her tweeting of pictures of Islamic State violence.
Reuters