The report on the Mediapart website and in Marianne magazine is "a shameless lie", the National Front (FN) leader told French radio. "I've never admitted anything to the investigators, whom I have never actually seen."
But Mediapart and Marianne reproduce part of the Olaf report stating that Le Pen had presented a short-term contract and a payslip for Thierry Légier to the European parliament for the period 1 October-31 December 2011 and that she later acknowledged that she "had not employed M Légier during these three months".
Le Pen argues that the money was actually payment due from an earlier period when Légier was employed as a European parliamentary assistant and that the parliament's pay office had accepted the contract as a means of correcting a previous error.
But the Olaf report also says she admitted that he had never even received the 41,554-euro salary.
The European parliament "never asks an MEP to produce fake documents or fake certificates to cover operations", Olaf said, insisting that the job was fake.
It has handed the case over to Paris prosecutors who have opened an investigation into fraud, breach of trust and undeclared employment.
Earlier contract probably fake
Olaf has also investigated Légier's earlier contract, in September-December 2009, when he was supposed to be working 12 hours a week at the European parliament while also acting as bodyguard to Le Pen's now-estranged father, Jean-Marie Le Pen.
While the 12,000-euro salary does seem to have been paid to Légier, the claim was probably made to use up Marine Le Pen's parliamentary allowance, Olaf believes, it having been "materially impossible" for Légier to do the same jobs simultaneously.
Parliament claims back 340,000 euros
The fraud office has reclaimed 340,000 euros from Marine Le Pen, both for the money paid to Légier and for the salary of Catherine Griset, whom it has found to have been paid as a parliamentary assistant while in fact working for the National Front at its headquarters in France.
Le Pen has refused to hand over the money, so it is due to be retained from her salary and allowances from this month on.
Still ahead in polls
The revelations do not seem to have harmed Le Pen's chances in this year's presidential election.
The latest opinion poll shows her ahead in the first round, with 26 percent of the vote against 20 percent for both centrist Emmanuel Macron and the mainstream right's François Fillon, whose ratings have suffered from allegations that he employed his wife and children as assistants in the French parliament.
All polls at the moment show whoever comes second would beat her in the deciding round, however.