The EU’s anti-fraud watchdog is seeking to recover €339,000 (£304,000) from the French far-right leader Marine Le Pen after an investigation into alleged misuse of European parliament funds, the French news magazine Marianne has reported.

The magazine said the watchdog, known as Olaf, had accused the Front National leader and French presidential hopeful of wrongly claiming the salaries of two parliamentary assistants, Catherine Griset and Thierry Légier, who it believed were in fact working almost entirely for the party.

The parliament was asked to recover the money in August and Le Pen was informed of the request in early September and again at the end of the month. Given until the end of October to respond, the magazine said, she had as yet failed to do so. The Front National did not immediately respond to a request for comment.

Le Pen’s lawyer, Marcel Ceccaldi, told Marianne she would be appealing to the European court of justice and would hold a press conference at the parliament in two weeks’ time to denounce what he described as an “instrumentalisation” of the rules around parliamentary assistants.

“How can you divide up the work of an MEP’s assistants into segments, slice their activities up into timed periods?” Ceccaldi asked.

He told the magazine and the investigative website Mediapart that the investigations were aimed at “restricting Marine Le Pen’s political activity at the parliament and censoring a critical voice by not allowing her the material means to exercise it”.

If Le Pen does not repay the €339,000 – the wages paid for the past five years to Légier, allegedly a Le Pen family bodyguard of 20 years’ standing, and Griset, who held several senior positions in Marine Le Pen’s office at the Front National HQ in Nanterre, outside Paris – the parliament could in principle begin withholding part of her MEP’s salary and expenses.

Other MEPs from the Eurosceptic, anti-immigrant party have been accused of similar abuses, including Le Pen’s father, Jean-Marie, and the Front National veteran Bruno Gollnisch, who in June were reported to be facing bills of €320,000 and €270,000. All have denied any wrongdoing.

The Olaf investigation and a parallel probe by French authorities were launched last year after the president of the European parliament, Martin Schulz, notified officials in Paris and Brussels that 20 parliamentary assistants hired by the Front National also held official positions within the party organisation.

The news comes at an unfortunate time for Le Pen, who is due to launch her campaign for the presidency early next year. Short of most of the several million euros needed fund a full campaign, the party has been relying on loans from Russian-backed banks that have reportedly not been extended.