Francois Fillon has said he would drop out of the race for the French presidency if he were criminally investigated.
Mr Fillon is at the centre of a media storm over allegations that for years his wife was paid for parliamentary work she did not do.
The conservative candidate told French TV there was nothing improper or illegal about his wife’s employment.
He said he would provide proof but he refused to be tried by the media.
The satirical weekly Le Canard Enchaine claimed on Tuesday that:
- Penelope Fillon earned about €500,000 (£430,000; $534,000) as her husband’s assistant in parliament, but there was no evidence she did the work
- She was paid a further €100,000 for a sinecure publishing job provided by a billionaire friend of her husband
That prompted financial prosecutors to open a preliminary investigation, though Mr Fillon and his wife will only be placed under formal judicial investigation or face charges if prosecutors find there is substance to the claims.