French conservative presidential candidate Francois Fillon on Wednesday accused the Socialist government of orchestrating a string of damaging leaks about his financial affairs in a bid to torpedo his election hopes.

The blistering attack from the Les Républicains candidate came as prosecutors widened a probe into whether he paid his British wife for a "fake" parliamentary job and amid reports of his alleged financial ties to the Russian government.

Slipping in polls that already predict he will be knocked out in the first round of voting on April 23, Mr Fillon went on the offensive to lambast weekly "organised leaks" which he said violated the confidentiality of the judicial investigation.

"Who are organising these? State services," he told Franceinfo radio.

"And, oddly enough, the Socialist party, Mr Macron and Mr Hollande pounce on these pseudo-revelations," he said, referring to election frontrunner Emmanuel Macron and Socialist President Francois Hollande.

Once in pole position in the race for the Elysée, Mr Fillon now stands to win just 17 per cent of the vote, according to an Elabe survey on Wednesday, with Mr Macron and the Front National's Marine Le Pen predicted to come top on 26 per cent. Mr Macron is expected to comfortably win the runoff.