The General Court of the European Union, the bloc's second-highest tribunal, rejected a bid by Le Pen to overturn a ruling that she had wrongly used parliamentary funds to pay an aide who was based at her party's headquarters in Paris.
The Luxembourg-based court said in a statement that it "confirms the decision of the European Parliament to recover from Marine Le Pen MEP almost 300,000 euros for the employment of a parliamentary assistant, on the ground that she did not prove the effectiveness of that assistant's work".
Le Pen, the leader of France's National Rally party -- known until a recent name change as the National Front -- provided no evidence "of any activity whatsoever on the part of the parliamentary assistant that comes under parliamentary assistance, which she moreover acknowledged during the hearing," the court said.
Le Pen swiftly announced her intention to take the case to the Court of Justice of the European Union, the bloc's highest court.
"This ruling is based not on the substance of the case but on a procedural aspect. We are going to appeal against this decision," she told the AFP news agency.
A source at the European Parliament told AFP that it has managed to recover around 60,000 euros from Le Pen by withholding the amount from her salary as an MEP before she quit the assembly in 2017.