Two French presidential hopefuls have snubbed former prime minister Edouard Philippe's call to join forces as centrists struggle to unite against the far right in next year's presidential election.

Polls show that far-right leader Marine Le Pen has her best shot of winning the Elysée Palace on her fourth attempt, as she benefits from one of the most fractured political landscapes in French history, with a multitude of candidates vying for support.

Philippe, who is seen as the candidate best placed to take on the far right, made a new push for unity on Tuesday, calling on forces from the right and centre to discuss a possible alliance in early November.

"If we continue to divide ourselves, we will not win," 55-year-old Philippe, President Emmanuel Macron's first prime minister, told reporters.

More than 30 declared or potential candidates from the left, centre and right are competing for voters outside the far-right and hard-left camps.