The announcement coincided with media revelations that Fillon's children transferred back to him large amounts of taxpayers' money that he also paid them, and news of a parallel inquiry by parliament's ethics ombudsman into a 13,000 euro ($13,800) gift of two suits Fillon accepted in February.

"We're cooked," a senior politician close to Fillon told Reuters before the news he had been placed under formal investigation.

"There's not a day goes by without more news. The suits saga is a disaster. It's something people can relate to. And now there's this story about reimbursements by the children."

Fillon is not the only candidate facing judicial probes. Le Pen, who is campaigning on a nationalist, anti-immigration platform faces allegations she underpaid taxes on a mansion that she and her father own.

Fillon's allies point to the steely far-right leader's refusal to attend judges' hearings. Le Monde newspaper reported on Tuesday that tax authorities were discussing with Le Pen a possible deal that would see her pay 63,000 euros in back taxes.

A lawyer for Le Pen declined to comment. Another lawyer was quoted by Le Monde as saying the deal was not done yet and that they still disagreed with tax authorities over its claims.

Unlike Fillon, however, opinion polls suggest Le Pen's anti-establishment campaign has not been damaged by the allegations against her.

Reuters