* Penelope Fillon faces allegations of being paid for minimal work
* Conservative candidate Fillon has slumped in polls
* Fillon accuses President Hollande of "dirty tricks" campaign (Adds Penelope Fillon being placed under investigation)
By Gérard Bon and Adrian Croft
PARIS, March 28 (Reuters) - The faltering campaign of French presidential candidate Francois Fillon suffered another setback on Tuesday when magistrates placed his wife under formal investigation over allegations that he paid her for a fake parliamentary job.
British-born 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 Fillon himself was placed under formal investigation in the same case.
The magistrates' decision brings the couple one step closer to a trial.
Fillon, 63, 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 Penelope Fillon had drawn about 500,000 euros ($540,000) in salary over eight years for working as an assistant to her husband when he was a lawmaker and later to the man who replaced him. It said there was little sign that she had done any work.
Fillon, a fan of late British leader Margaret Thatcher, has tumbled to third place in the opinion polls, which show him being eliminated in the first round of the election on April 23.
While denying he did anything illegal, Fillon has conceded he made errors of judgment, 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.
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.
Penelope Fillon, 61, made no comment on Tuesday. 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 May 7.
An Ipsos poll on Tuesday was the latest of many to show Macron and Le Pen six or seven percentage points ahead of Fillon in the first round.
Macron, a former investment banker, said on Tuesday he expected his untested new party to be able to form a parliamentary majority after legislative elections in June if he wins the presidency in May. ($1 = 0.9250 euros)
(Additional reporting by Sudip Kar-Gupta, Michel Rose and Simon Carraud; Writing by Brian Love and Adrian Croft; Editing by James Dalgleish)