The European Court of Justice confirmed French far-right leader Marine Le Pen will have to reimburse €300,000 to the European Parliament on the grounds that she did not prove the effectiveness of her parliamentary assistant’s work.

Marine Le Pen was appealing a previous ruling by the General Court of the European Union of June 2018, sentencing her to pay back the sum for parliamentary assistance to the Strasbourg institution.

"The appeal is dismissed," the European Court of Justice ruled earlier this week in an ordinance obtained by Euronews, and "Le Pen is ordered to pay the costs."

Marine Le Pen took to Twitter on Wednesday to express her indignation and announce that her legal battle wasn't over.

"I have produced all the proofs of my assistant's work and the European Court of Justice refused to examine them twice," Le Pen wrote.

"The violation of my rights is obvious. I am taking the case to the European Court of Human rights," she continued.

"Our response to these disgraceful manipulations will be reflected in the ballot boxes," the National Rally leader added as France is set to vote for European elections this Sunday.

Le Pen is hoping to strike a blow to Emmanuel Macron's presidency by overtaking his centrist, pro-European party Republic on the Move.

Polls currently give her National Rally party a slight lead.