The EU's European Anti-Fraud Office (Olaf) has confirmed reports by French website Mediapartand Marianne magazine that it is seeking 339,946 euros from Marine Le Pen after completing an investigation in July.
The sum is the equivalent of the salaries of two assistants, Catherine Griset and Thierry Légier.
Assistant, chief of staff, bodyguard
Griset, a long-time friend of Le Pen who also used to be her sister-in-law, was an accredited parliamentary assistant to the FN leader between 2010 and 2016, supposed to work in the European Parliament in Strasbourg and Brussels.
But at the same time she held administrative posts for Le Pen at the party's headquarters in Nanterre.
Légier, a former parachutist who was bodyguard to Le Pen's father, Jean-Marie, for 20 years before performing the same service for Marine, was a "local assistant" for three months in 2011.
He was also a regional councillor in Normandy.
Socialist plot alleged
Le Pen will refuse to cooperate with the inquiry unless she receives the full Olaf report, her lawyer Marcel Ceccaldi said.
"How can you possibly separate the activities of an MEP and the president of a major political party?" he wanted to know on Monday.
FN vice-president Florian Philippot on Tuesday accused the EU of "soft on its mates, with Europeanists, and pitiless with its adversaries", pointing to an ethics panel's decision to take no action against commissioner Barroso for taking a job with Goldman Sachs while questioning his judgement in doing so.
In 2015 Le Pen herself accused European Parliament President Martin Schultz, a member of Germany's Social-Democratic Party, of working with France's ruling Socialists to persecute her party after he alerted Olaf to potential irregularities in the payment of 20 FN parliamentary assistants.
FN pays 97 people from EU funds
The party's representation in the European parliament leapt from three to 23 in the 2014, giving it access to substantial finance, regardless of its low opinion of EU institutions.
Each MP has a monthly salary of 6,400 euros, an expense allowance of up to 4,320 euros and a budget of up to 23,932 euros to pay assistants, allowing it to pay 97 people, according to Mediapart.
Twenty of them also hold or have held administrative positions for the party and the contracts of 29 name Nanterre as their place of work.
Other parties accused
Mediapart has accused several other parties, including the Socialists, of using parliamentary assistants for purposes not strictly linked to their function.
But "with the FN it seems we're not talking about marginal fiddling but a massive, systematic, industrial-scale operation", former European parliament vice-president Gérard Onesta, who helped draw up the institution's rules, told the website.
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