PARIS (AFP) - Former French prime minister Alain Juppe ruled out a run for the presidency on Monday (March 6), boosting embattled rightwing partycolleague Francois Fillon whose campaign has been thrown into chaos by a fake job scandal.

Juppe, 71, was the most likely candidate to replace Fillon and try to unite their deeply divided Republicans party only seven weeks from the start of the two-stage election.

Polls suggest Juppe would be more popular with voters, but the centrist is considered too soft on immigration and other social issues for many of Fillon's supporters on the right flank of the party.

"I confirm for a final time that I will not be a candidate to be president of the republic," Juppe said in a downbeat statement that criticised Fillon and said France was "sick" and suffering from a "profound crisis of confidence".

His decision removes a major rival for Fillon, who is sticking with his bid for power despite the prospect of criminal charges later this month, mounting criticism within the party and falling poll numbers.

The conservative 63-year-old was once the favourite to be France's next leader but his campaign is mired in accusations he used public funds to pay his wife hundreds of thousands of euros for fake parliamentary jobs.

"No one today can prevent me being a candidate," Fillon told France 2 late on Sunday, emboldened by a rally of tens of thousands of supporters in Paris earlier in the day.

Party leaders are to meet for crisis talks on Monday evening, with allies of ex-president Nicolas Sarkozy still pushing Fillon to step aside and name a replacement.

This infighting and Fillon's chaotic campaign have made an already unpredictable election even harder to call.

The disarray appears to have benefited centrist pro-business candidate Emmanuel Macron in particular, as well as far-right leader Marine Le Pen, who are shown by polls as the top two candidates in the first round on April 23.

Polls suggest 39-year-old Macron would beat Le Pen in the decisive second round on May 7 - but after Donald Trump's victory and Britain's vote to leave the European Union, analysts caution against bold predictions.

"Never under the fifth republic have we had an election in such confused conditions," Juppe said, stressing the dangers of National Front leader Le Pen's "anti-European fanaticism" and Macron's "political immaturity".

Fillon's defiance and accusations that the government, justice system and media were plotting against him have "led him into a dead-end", Juppe added in one of several criticisms of his colleague.

Both Le Pen and Macron - one a far-right anti-establishment figure, the other an independent who founded a new political movement last year - have tapped into widespread anger at France's political class.

"French people want a profound renewal of their politics," Juppe, a veteran politician with a conviction over a party finance scandal, told a press conference in his hometown of Bordeaux.

"Evidently I do not embody this renewal," he added.

National Front vice-president Florian Philippot said Monday that many French people who were thinking of voting Fillon would now opt for Le Pen.

"They want a free, safe and prosperous France, not a France that is subjected to the most brutal winds of globalisation," he told LCI television.

Current President Francois Hollande also warned in an interview published Monday that the threat of a Le Pen presidency was real but that he would fight to prevent it happening.

France "is aware that the vote on April 23 and May 7 will determine not only the fate of our country but also the future of the European project itself," he added.

Fillon, a devout Catholic, beat Juppe in the Republicans' primary in November, pulling off a surprise victory by campaigning as a "clean" candidate.

He was the frontrunner in the presidential race until Le Canard Enchaine newspaper revealed in late January that he paid his wife Penelope and two of their children nearly 900,000 euros (S$1.3 million) as his parliamentary assistants.

Ahead of the meeting of Republicans party leaders later Monday, former president Nicolas Sarkozy piled pressure on Fillon to meet Juppe and find a way out of the crisis.

Sarkozy urged the two men to meet "to find a dignified and credible way out of this situation which cannot continue and which is creating serious problems for the French people".

A number of Sarkozy's closest allies have already called on Fillon to step aside.