For weeks, Mr Fillon has fought allegations that his wife was paid for years for work she did not do.
Mr Fillon called the investigation "a political assassination" against him.
President Francois Hollande criticised his words, and accused him of attacking France's judicial system.
"Being a presidential candidate doesn't authorise you to cast suspicion on the work of police and judges," Mr Hollande said.
Mr Fillon says he has been summoned to appear before the judge, Serge Tournaire, on 15 March.
The date is just two days before the deadline for candidates to submit their final applications. The first round takes place on 23 April, followed by a second-round run-off on 7 May.
Mr Fillon, who represents the centre-right Republicans party, says it is up to voters to decide his fate.
"It's not just me that is being assassinated, it's the presidential election. The voices of millions of votes have been muzzled," he said on a speech on Wednesday.
"The closer we get to the date of the presidential election, the more scandalous it would be to deprive the right and centre of a candidate," he argued.
He has faced criticism for his decision - especially because he had previously pledged to stand down as a candidate if his case was placed under formal investigation.
Mr Fillon's foreign affairs spokesman, Bruno Le Maire, has resigned from the campaign team, saying (in French) he has undermined the "credibility of politics".
One allied party, the centre-right UDI, says it is suspending support for his campaign while it considers whether to still back him.
Mr Fillon was selected late last year in national primaries held by the centre-right Republicans that attracted some four million voters.
For a time he was the favourite in the race to succeed Francois Hollande as president - until the fake job allegations emerged.
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