By Richard Balmforth

PARIS (Reuters) - An investigation into allegations of "fake work" against the wife of French presidential frontrunner Francois Fillon may be harming his popularity, an opinion survey suggested on Friday.

The conservative former prime minister was the clear favorite to win a presidential election in three months' time until a satirical weekly said this week his wife had received big salaries from jobs where she did no actual work, including several years as his parliamentary assistant.

The affair has dented a wholesome image that Fillon, 62, a devout Catholic with 30 scandal-free years in politics, has sought to project. He has said he would abandon his election run if placed under formal criminal investigation.

Though it is legal in France for parliamentarians to employ family members, financial prosecutors have opened an investigation into suspected misuse of public funds.

Fillon made a spirited defense of both himself and his wife, Penelope, on television on Thursday night, describing the allegations as an "abject" attack aimed at harming his presidential bid.

An Odoxa poll, carried out online on Thursday before that TV appearance, showed a four-point popularity drop since its last survey on Jan. 8.

Harris Interactive separately showed a drop of a single percentage point from December, with views collected in the days immediately following the report.

Most political analysts see Fillon winning a run-off ballot against far-right leader Marine Le Pen.

Polls give him a majority of around two-thirds that would be hard to erode with barely three months to go to the predicted showdown on May 7.

But also making a strong poll showing is centrist independent Emmanuel Macron, in third place. His political fortunes could depend on who wins the Socialist ticket at a primary on Sunday.

A victory by leftwinger Benoit Hamon over ex-prime minister Manuel Valls, which is expected, could lead centrist-minded Socialist voters to support Macron.

Le Canard Enchaine weekly said Penelope Fillon had been paid roughly 600,000 euros ($640,000) for employment by him and his successor in parliament and later as a literary reviewer for a cultural journal. Fillon did not deny the figure.

The Canard Enchaine said there was little sign Penelope had done any work in any of the jobs. Fillon brushed this aside, saying she had worked for him ever since he entered politics.

"Without the work my wife carried out I would not be where I am now," he said.

In his television appearance, Fillon also said he had paid two of their children for work as lawyers when he was a senator between 2005-7. Marianne magazine suggested this was questionable conduct since the children, though studying law, were not fully qualified at the time.

Financial police investigators on Friday met Michel Crepu, the former director of the literary review for which Penelope Fillon was reported to have worked for 10 months. Crepu has said he never knew Penelope Fillon worked for the review.

(Additional reporting by Simon Carraud, Gerard Bon and Ingrid Melander; Editing by Andrew Callus and Robin Pomeroy)