Mr Fillon, whose unexpected win in last year's Republican primary propelled him into the limelight, has failed to garner support from conservative voters, whom are feeling increasingly underwhelmed by his radical political ideas, a project Mrs Le Pen says will destroy France.
Carried out by the Cevipof research institute for the French daily Le Monde, the poll, which focuses on actual voting intentions, shows that Mr Fillon's support has dropped steadily since his surprise victory in November.
In December, a Cevipof poll showed that Mr Fillon was poised to breeze through the first round of voting with 26 to 29 per cent of voting intentions, but in this new poll, he received 23 to 25 per cent of voting intentions, and is, as a result, he is no longer a firm favourite.
Right-wing voters in France prefer presidential candidate Marine Le Pen (R) over Francois Fillon (L)
Mrs Le Pen, however, has surged ahead of Mr Fillon with 25 to 26 per cent of voting intentions.
If this poll is right, she will make it to the second round of the election and could very well become France's first female, right-wing president.
"Mrs Le Pen's supporters are incredibly loyal and would never betray her. Mr Fillon's supporters, on the other hand, are more fickle, and quickly became disenchanted with his hardline policies," political scientist Pascal Perrineau told Le Monde.
Francois Hollande took over from Nicholas Sarkozy in 2012
In 2012 – the year Francois Hollande, France's deeply unpopular president, took over from Nicholas Sarkozy – Mrs Le Pen was credited with 19 to 20 per cent of voting intentions, but this new poll shows that her party, the anti-EU and anti-immigrant National
Front, is gaining ground in France.
Socialists have no chance of beating Mrs Le Pen, the poll shows: former industry minister Arnaud Montebourg has been credited with seven per cent of voting intentions in the first round of voting, and so has former education minister Benoit Hamon.
Marine Le Pen’s party National Front is gaining ground in France, says a new poll
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FACTS about Marine Le Pen
Wed, November 23, 2016
Marine Le Pen is a French politician who is the president of the National Front, a national-conservative political party in France and one of its main political forces.
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A French politician who is the president of the National Front (FN), a national-conservative political party in France and one of its main political forces.
Former prime minister Manuel Valls has been given 10 per cent of voting intentions in the poll.
'Rising star' Emmanuel Macron – Mr Hollande's one-time protege -, for his part, has been credited with 17 to 20 per cent of voting intentions, and is the only left-winger tipped to make it to the final round runoff next May.
The poll was conducted between the 10 -15 January, and 15,921 people aged 18 or over, were interviewed.