Fillon has been convinced that the investigation into alleged misuse of public funds linked to allegations his wife Penelope earned hundreds of thousands of euros as a "fake" parliamentary assistant would soon be dropped and allow him to concentrate on his battle to become the next president.
But on Thursday France's specialist prosecutors raised the prospect of Fillon being charged over the affair when they released a statement saying the investigation could not be dropped.
"It is my job to say that the number of elements already gathered do not permit us, in the current state, to drop the procedure," read a statement from the specialist financial prosecutor.
According to L'Express newspaper that means prosecutors have two options: to formally open a judicial investigation against François Fillon or to send the case straight to a criminal court.
Prosecutors could still decide to drop the case at a later date if they eventually decide there is insufficient or little evidence to suggest wrongdoing.
Fillon himself reacted angrily to the statement from the prosecutor insisting there was "nothing new" and that his determination to modernise France through his promised policies was as strong as ever.
Fillon, whose popularity has plummeted since the start of the Penelopegate affair last month, has doggedly refused to step down despite pressure from within his party.
Although he admitted it was an error to employ his wife as his parliamentary assistant, for which she was paid an average salary of €3,600 net, he insists he has done nothing illegal, preferring to point the finger at the press for what he called a media lynching.
He has presented himself as the victim of a dirty tricks campaign which he claimed was orchestrated by the left to prevent the right regaining power after five years of Socialist rule.
However he has repeatedly said he would stand aside if he was "mis en examen" which means formally investigated and is often equated to "charged" in English.
"Investigations will continue with strict respect for the rules that govern the penal code," the prosecutor added.
After the scandal broke on January 25th Fillon asked his supporters in his Republicains party to give him two weeks in order to clear his name. However the calls that he has faced to step down will likely grow after the news thatthe investigation will rumble on with just over two months to go before first round of the presidential election.
After winning November's centre right primary Fillon was immediately installed as clear favourite to become the next president, but since Penelopegate broke has tumbled in the polls and now faces missing out on the second round run off with Marine Le Pen and Emmanuel Macron likely to face off against each other.