Officials filed preliminary charges against Fillon in a probe of the publicly funded jobs arranged for his wife and children - jobs they allegedly never did.
Mr Fillon attempted to draw a line under corruption allegations this week, and focus on his programme, which he detailed on Monday.
"She corrected my speeches, she received countless guests, she represented me in protests, she passed on people's requests ... she did it willingly for years", Mr Fillon said in his wife's defence.
At his request, Mr Fillon met the investigative judges on Tuesday morning, one day earlier than the date set for the meeting, to avoid media attention, a spokeswoman for his campaign said. So while the announcement means he will be forced onto the backfoot again on the campaign trail, Bruno Jeanbart, deputy director of pollsters OpinionWay, said voters have probably already factored the charges into their thinking.
However, Mr Fillon appeared to be narrowing the gap with Mr Macron on Tuesday despite his legal woes.
Fillon has been accused of several offenses, including misusing public funds and misuse of corporate assets, his lawyer said.
This is the moment that Francois Fillon feared, but which he knew was probably coming, the BBC's Hugh Schofield in Paris reports.
"Those who don't respect the laws of the Republic should not be allowed to run".
The weekly Le Canard Enchainé newspaper originally reported the allegations about Fillon's employment of his relatives.
Parliament's ethics chief has also revealed it is to open an inquiry into how he came to accept a gift of two bespoke suits worth £11,380 last month, in the latest twist in the tale.
But it's not all rosy.
Business France, a French public agency, is suspected of choosing French communications group Havas for organizing the event without seeking a bid.
Another top contender has also caught the attention of judicial investigators. Where once he was the odds-on favorite, Fillon has lately placed third in polls - behind Marine Le Pen, of the far-right National Front party, and independent centrist Emmanuel Macron.
Polls suggest 39-year-old Macron would beat Le Pen in the decisive second round on May 7 - but after Donald Trump's victory in the United States and Britain's vote to leave the European Union, analysts caution against bold predictions.