France 24 says Le Pen "has so far refused to attend a police interview over the investigation... citing her status as a member of the European parliament".
French judges had been investigating le Pen after she tweeted three images of Isis executions in December, including one of the American journalist James Foley who was beheaded by the terror group in Syria.
Fillon was the frontrunner until mid-January when "Le Canard Enchaine" newspaper alleged he paid his Welsh-born wife Penelope and two of their children almost 900,000 euros as parliamentary assistants or advisors.
Another poll, by Odoxa, on Friday showed Macron finishing ahead of Le Pen in the opening round.
"The thing about the judicial affairs for Marine Le Pen and the National Front is that they are not about personal enrichment, while Francois Fillon's family is directly implicated", far-right expert Cecile Alduy from Stanford University told a conference this week.
Conservative candidate Francois Fillon has come under fire over allegations that his wife and children were given parliamentary jobs.
Le Pen, campaigning on an anti-immigration and anti-EU platform, is forecast to reach the second round of an election that is influenced by the same populist themes that propelled US President Donald Trump to power and led to the Brexit vote in Britain.
Despite the fraud, allegedly costing more than €300,000 to the European taxpayer, she has recently increased her lead in the first round polls, paradoxically mainly because of Fillon's own fake jobs accusation.
The attack on Macron diverted attention away from a political discussion that has emerged in France regarding Francois Fillon, the right wing political candidate.
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The Union of Democrats and Independents (UDI) called on Mr Fillon's Republican Party to choose another candidate in the wake of a scandal over payments.
"It's a bit much.so she'll respond after [the elections]", he said.
As a centrist party, it broadens the Republicans' base, he said.
Reuters reported that Le Pen has lost her diplomatic immunity before, in 2013, also over comments she made about Muslim people.
Sources in the Fillon camp have said there was a strong push by some in the party on Wednesday to have former prime minister Alain Juppe stand in Fillon's place, but that the plan was vetoed by Nicolas Sarkozy, the former president.
Even more significantly, Ms. Le Pen's chances of sweeping to victory may have been checked by the rise of Emmanuel Macron, a charismatic 39-year-old former banker who describes himself as a radical centrist and who this week took over first place in a poll.
Before the allegations surfaced, Fillon was the clear leader in opinion polls.
The poll of voting intentions by research firm BVA found Le Pen garnering 26 percent of the votes in April's first round, down 1.5 percent from the last BVA poll on February 23.