The legal woes of Fillon, a conservative who had led the race for weeks, have benefited centrist Emmanuel Macron in a tightening contest.
Fillon's problems are the latest twist in an election being watched closely across Europe after the victory of the Brexit camp in Britain and Donald Trump's win in the United States.
A poll yesterday showed that Marine Le Pen, the leader of the anti-immigration and anti-EU National Front (FN), would score highest in the first round on April 23, followed by Fillon and Macron almost neck-and-neck.
Fillon and his Welsh-born wife Penelope were quizzed separately by investigators over claims in the Canard Enchaine newspaper that she received payments for several years as his parliamentary aide.
While lawmakers are entitled to employ family members, the paper said it could find no proof she did any work in return for the money.
The couple were also questioned over money received for Penelope's work at a literary review owned by a billionaire friend of Francois Fillon.
"They're trying to take me down, through Penelope," he said.
But the scandal has damaged the 62-year-old former prime minister who was a surprise winner of the rightwing nomination after campaigning as a "clean" candidate who could slash public spending by cutting the jobs of 500,000 civil servants.
The deeply divided ruling Socialist party chose the hard-left Benoit Hamon as their candidate yesterday.
Hamon, 49, was a rank outsider just three weeks ago but leftwing supporters weary after President Francois Hollande's troubled five years in power warmed to his radical programme that includes paying everyone a universal basic income rising to 750 euros (USD 800) a month.
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