PARIS — France’s financial prosecution office opened an embezzlement investigation Wednesday into François Fillon, a leading presidential candidate, following a newspaper report that his wife had been paid around $535,000 in public money for a no-show job, a revelation that could upend the tightly contested election.

Mr. Fillon, who won November’s center-right primary, has been considered a favorite in the race, but now must contend with questions about whether the payments to his wife were inappropriate. According to the satirical weekly The Canard Enchaîné, Mr. Fillon’s wife, Penelope, received about 500,000 euros over eight years, first as his parliamentary assistant and then as assistant to his deputy Marc Joulaud, who took over when Mr. Fillon became a minister in the government in 2002.

It is not illegal in France for members of Parliament to employ family, and around 10 to 15 percent do so, according to French media. But if Mrs. Fillon did nothing, holding a fake job, in effect, in return for the public funds, her husband could be in trouble.

“This could be painful to Fillon, fairly damaging,” Gérard Grunberg, a political scientist at Sciences-Po, the prestigious political science institute, said Wednesday in an interview. He added: “Everything depends on whether it was a bogus job. If it’s proved that it was — and a well-paid one, too — then this is going to be important. She always said she didn’t participate in political life. So she sort of condemned herself in advance.”