The investigation for "misuse of public money" was triggered by a report in the Canard Enchaine newspaper.
The investigation for "misuse of public money" was triggered by a report in the Canard Enchaine newspaper, which claimed that British-born Penelope Fillon was paid from money available to her husband as an MP for the northern Sarthe region.
It is not illegal for French parliamentarians to employ family members, providing the person has a genuine role.
At issue is what work Penelope did to earn a salary of sometimes around 7,000 euros a month between the late 1990s and the late 2000s.
The paper said it had been unable to find anyone who remembered her working in parliament.
Fillon, a conservative who is leading the race to become president in May with promises to slash public spending, has dismissed the controversy as "mudslinging."
His wife has kept a low profile throughout Fillon's nearly four-decade political career.
The former prime minister himself has said his wife raised the first four of their five children virtually single-handedly at their chateau in Sarthe, while he was a lawmaker in Paris.