French presidential candidate Francois Fillon has apologised for the "error" he made in hiring his wife as a parliamentary aide while denying she was paid for a fake job.

"I apologise to the French people," the conservative candidate told a press conference, admitting it was an "error" that he regretted "deeply".

In an aggressive performance that saw him accuse the media of trying to destroy him politically, the 62-year-old former prime minister said his British-born wife Penelope's salary was "perfectly justified" and he would continue in the presidential race.

Addressing allegations that Penelope did not actually perform any duties to earn hundreds of thousands of euros, Mr Fillon said: "No-one has the right to judge what a parliamentary assistant's job consists of, except the MP himself."

He said his wife had worked constantly to assist in his constituency business and that her average monthly salary of €3,700 after tax over 15 years had been fair for a woman with training in law.

He said: "I am the only candidate who can bring about a national recovery. I am the candidate of the Right and I am here to win." 

Mr Fillon announced that he would launch a new phase of his campaign from tomorrow. 

He came under mounting pressure to quit the race over the past fortnight since a newspaper alleged his wife was paid hundreds of thousands of euros in state money for work she may never have done.

Even some senior members of his own party have told him to stand aside for someone else in time to build a campaign for a vote that is now just 11 weeks away.

Opinion polls show Mr Fillon lost his status as favourite to win the presidency to independent centrist Emmanuel Macron, and that far-right leader Marine Le Pen has also gained ground.

He said at the weekend he would fight to the end to defend his position as the party's nominee.