French presidential candidate Francois Fillon has been placed under formal investigation over allegations of diversion of public funds and misappropriation of money.

The prosecutor had been conducting a preliminary probe into allegations that Fillon's wife Penelope received more than 900,000 ($1.38 million) for a "fake job" as parliamentary assistant to her husband and his successor over several years.

She also faces allegations that she was unfairly paid 100,000 by a magazine called La Revue des Deux Mondes, owned by a close friend of her husband, for writing a handful of short articles. Two of their children are suspected of being being paid more than 80,000 for fake jobs as parliamentary aides.

Penelope Fillon is due to be summoned by judges later this month.

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Both have denied any wrongdoing.

Francois Fillon is also under investigation for "failure to make compulsory declarations to the higher authority for transparency in public life". This concerns an undeclared loan of 50,000 from a friend.

He was initially due to meet judges overnight.

But his lawyer, Antonin Levy, said: "The hearing [to place him under investigation] was brought forward so that it could take place in calm conditions."

In the French justice system, being placed under investigation does not confirm wrongdoing but means investigators have serious grounds for pursuing the matter.