The European Court of Justice based in Luxembourg rejected an appeal from Le Pen, a former MEP who had contested a lower court ruling, according to a copy of the decision obtained by AFP on Thursday.
The top court upheld a June 2018 General Court ruling which confirmed “the decision of the European Parliament to recover from Marine Le Pen MEP almost €300,000 for the employment of a parliamentary assistant.”
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The lower court, also in Luxembourg, cited grounds that Le Pen “did not prove the effectiveness” of that assistant's work.
In December 2016, the parliament ruled that €298,500 had been “unduly” paid to Le Pen over a five-year period for parliamentary assistance provided by her aide Catherine Griset, who was based in Paris and not at the legislature.
The EU parliament meets in both Brussels and the French city of Strasbourg.
The parliament began withholding part of Le Pen's allowances as an MEP to recover the money, but she stood down in 2017 and is so far down her party's list in this week's European elections she is unlikely to return.
While campaigning for Sunday's polls, Le Pen denounced “the flagrant violation” of her rights and vowed to take the case to the European Court of Human Rights in Strasbourg.
Le Pen added: “Our response to these low manoeuvres will be seen at the polling booths.”