Marine Le Pen, the leader of the Far-Right National Front, is ahead in the polls as France heads towards the first round of presidential elections in the spring.
Polls put her on 26.5 percent, ahead of Mr Fillon, a conservative former prime minister, who is on 25 percent.
But only two candidates make it into the second round of the presidential election in May and Ms Le Pen is expected to be defeated ultimately by Mr Fillon, with most Socialists expected to prefer him as the lesser of two evils.
In 2002 Ms Le Pen’s father Jean-Marie, who founded the National Front, made it into the second round, pipping the Socialist Lionel Jospin, but was thrashed by 82 percent to 17 percent by conservative Jacques Chirac, who garnered support from many on the Left.
The ruling Socialists are set to choose their candidate later this month – Manuel Valls, who stepped down as prime minister last month is the frontrunner but is almost as unpopular as outgoing President Francois Hollande.
Emmanuel Macron, a former economy minister who is running as an independent, is currently estimated to get 17 percent of the vote in the first round.
The latest poll, by Ifop Fiducial for Paris Match, suggests Mr Fillon would beat Ms Le Pen by 64 percent to 36 percent in the second round.
But if Mr Macron made it to the runoff he would actually beat Mr Fillon by 52 percent to 48 percent.
The poll, which had a margin of error of 1.3 percent, was conducted last week and was based on a sample of 1,860 people registered to vote.
Mr Fillon, who is battling it out with Ms Le Pen for right-wing votes, said today France needs to introduce immigration quotas, although it was not clear how he proposed to do this while remaining inside the European Union.
Before a visit to Menton, on the Italian border, Mr Fillon said: ‘I want France to be able to decide every year the number of people it can accept on its territory.’