Former French prime minister Alain Juppe has said he is backing Francois Fillon's presidential bid and would be one of his official sponsors, only days after slamming his fellow conservative in a speech.

"Even if just a passenger, I'm not leaving the ship during the storm," Mr Juppe said on his Twitter account.

Mr Juppe, who was defeated by Mr Fillon in the centre-right primaries in November, had been seen by many conservatives as a potential 'plan B' since the 63-year-old became embroiled in a political scandal.

Earlier this week, Mr Juppe had uncharacteristically harsh words for Mr Fillon, saying he had wasted the Republican party's strong lead in the opinion polls and calling him obstinate.

Candidates need at least 500 official sponsors to compete in the presidential election in April and May. Mr Fillon already has over 1,000, according to the constitutional council's website.