France's national financial prosecutor has opened a preliminary inquiry into the claims that she was paid from an allowance available to her husband.
Fillon said in a TV interview Thursday that he was "disgusted" at allegations that Welsh-born Penelope Fillon did nothing to earn half a million euros ($534,000) paid from public funds over more than a decade.
He said his wife had "always worked for me" during his four-decade political career and listed tasks she had carried out, including "editing my speeches" and representing him at events.
France's national financial prosecutor has opened a preliminary inquiry into the claims that she was paid from an allowance available to her husband as a member of parliament.
A new opinion poll Friday showed that the popularity of Fillon, a former prime minister who is widely seen as the frontrunner in the presidential contest, had taken a hit over the allegations.
Thirty-eight percent of voters across the political spectrum have a "good opinion" of him in the wake of the claims about his wife's jobs, a drop of four points from a poll on January 8, and 16 points less than in early November, the Odoxa poll of 1,012 people showed.
Le Canard Enchaine, the newspaper that made the claims, also alleged that in 2012 and 2013, Penelope Fillon had a second paid job at a literary periodical, La Revue des Deux Mondes, which is owned by a billionaire friend of Fillon, Marc Ladreit de Lacharriere.
The former publisher of La Revue des Deux Mondes, Michel Crepu, said this week that while Fillon's wife had contributed comments to several articles, "I do not have the slightest indication of what could be described as a job as a literary consultant."
Crepu was questioned Friday by investigators probing the case.
If Fillon does drop out of the race, the man he beat to the rightwing nomination last year, veteran centrist Alain Juppe, on Friday "clearly and definitively" ruled out any suggestions he could step in.
In a race being watched closely after Britain's vote to leave the EU and Donald Trump's victory in the United States, Fillon is currently forecast to reach the presidential runoff in May, with far-right candidate Marine Le Pen his most likely opponent.
But centrist Emmanuel Macron is increasingly in contention, having attracted packed crowds to campaign events.
On Sunday, former premier Manuel Valls and leftwing radical Benoit Hamon will fight it out for the nomination of the governing Socialist party.
The leftwing primary has become a battle of two factions within the Socialist party, with Valls's reformist agenda clashing with Hamon's attempts to claw back the party's "true" ideals.
Whoever wins, he will face an uphill task, with polls showing the Socialist candidate will be eliminated in the first round as the party pays the price for five years of economic stagnation under deeply unpopular President Francois Hollande.
Hamon has made headlines with a proposal to adopt a universal basic income -- a state handout of around 750 euros a month, paid to everyone.
He would pay for it partly by taxing the wealth created by robots, and abandon a labour law introduced last year that made it easier to hire and fire employees.
Valls has argued that the basic income plan would "ruin" France and that a Hamon victory would spell "certain defeat" in the presidential race.