French presidential candidate Francois Fillon, an outsider to win after involvement in financial scandal, said on Monday he would order a parliamentary inquiry into allegations President Francois Hollande interfered in the justice system, if elected.

Once the frontrunner, the conservative former prime minister's poll ratings have slumped since allegations surfaced that he paid his wife, a son and a daughter hundreds of thousands of euros of public money for minimal work.

Although some polls show his support recovering slightly with less than three weeks to the 23 April first round, he is well behind far-right leader Marine Le Pen and independent centrist Emmanuel Macron, who are tipped to go through to a 7 May run-off.

Mr Fillon, 63, who is being investigated by magistrates over the jobs allegations and over a gift of expensive suits, insisted on his innocence.

"If I had the slightest doubt about my guilt I wouldn't be a candidate in the presidential election," he told BFM TV.

He said he was the victim of "manipulation" and believed his case was being closely followed "by the highest authorities".

He drew back from previous allegations that Mr Hollande, a Socialist president who is not standing for a second term, had personally led a smear campaign against him. He said he could not prove this.

Fillon announces he will not be standing down from the French presidential race

He said however that prosecutors should open an inquiry into allegations made in a book by two journalists from the satirical weekly Le Canard Enchaine that Mr Hollande had had judicial wiretaps that interested him sent to his office.

"Prosecutors should take up this case. If they don't do so and if I am elected president, there will be a parliamentary commission of inquiry," Mr Fillon said.

Mr Hollande's office has rejected Mr Fillon's accusations and denied interference in the justice system.

Mr Fillon said that, given the investigations against him, there was "every chance" his own phone was tapped.

Mr Fillon also said that Francois Baroin, a former finance minister, would be a "very good choice" for prime minister if he won election to the Elysee.

Investors are concerned the wave of frustration with political elites behind Britain's vote to leave the European Union and the election of Donald Trump in the United States could sweep Ms Le Pen into power in France.

The National Front leader would take France out of the euro and hold a referendum on EU membership.

The latest poll, by Opinionway, showed Ms Le Pen, with 25 per cent in the first round, one point ahead of Mr Macron and six points ahead of Mr Fillon.

It showed Mr Macron would easily beat Ms Le Pen in the run-off, but former Prime Minister Manuel Valls said last week he believed Ms Le Pen's potential score was seriously underrated.

Mr Macron told Le Monde in an interview on Monday that Mr Fillon and Socialist candidate Benoit Hamon were wrong to focus their attacks on him, when Ms Le Pen was the real threat.

"Those who say Marine Le Pen can't win the second round are the same as those who said Trump could never win. We don't know what can happen if she is far ahead in the first round," he said.