The campaign of French presidential candidate Francois Fillon suffered another setback when magistrates placed his wife under formal investigation over allegations that he paid her for a fake parliamentary job.

Penelope Fillon will be investigated on suspicion of complicity in misappropriating public funds and several related offences, a judicial source said.

The decision, announced after she was questioned by magistrates, comes two weeks after Mr Fillon himself was placed under formal investigation in the same case.

The magistrates' decision brings the couple one step closer to a trial.

Mr Fillon, the main conservative candidate in France's presidential election being held over two rounds in April and May, had been favourite to win until the fake job allegations surfaced in late January.

The allegations were first reported by satirical weekly Le Canard Enchaine, which said Ms Fillon had drawn about €500,000 in salary over eight years for working as an assistant to her husband when he was a politician and later to the man who replaced him.

It said there was little sign that she had done any work.

Mr Fillon has tumbled to third place in the opinion polls, which show him being eliminated in the first round of the election on 23 April.

While denying he did anything illegal, Mr Fillon has conceded he made errors of judgement, both in the "Penelopegate" affair and in accepting expensive made-to-measure suits as a gift from a lawyer renowned for his role as a deal-maker in Africa.

Mr Fillon has accused Socialist President Francois Hollande of waging a "dirty tricks" campaign against him and he has resisted pressure from within his own party to pull out of the campaign.

Ms Fillon made no comment today. She said in a newspaper interview this month that the work she had carried out for her husband was real.

"He needed someone that carried out his tasks," she told the Journal du Dimanche newspaper.

"If it hadn't been me, he would have paid someone else to do it, so we decided that it would be me."

Polls show centrist Emmanuel Macron would trounce far-right leader Marine Le Pen in France's presidential election if, as seems increasingly likely, the two face off in the final round of the contest on 7 May.

An Ipsos poll today was the latest of many to show Mr Macron and Ms Le Pen six or seven percentage points ahead of Mr Fillon in the first round.