The conservative ex-prime minister will speak at his campaign headquarters in Paris at 1500 GMT.
The conservative ex-prime minister, who has refused to step aside over the scandal stemming from payments to his wife and children, will speak at his campaign headquarters in Paris at 1500 GMT.
"Mr Fillon wants to address the French people," one of the sources said, adding he would be "combative".
The rightwing Le Figaro newspaper said it would mark the start of a "counter-offensive".
Fillon "will explain the facts and say that he is continuing the fight," an unnamed Fillon aide told the paper, adding: "He will not give up."
Fillon's presidential bid has been floundering since it emerged that his British-born wife Penelope was paid over 800,000 euros ($860,000) from public funds for a suspected fake job.
He also paid two of his children to act as his assistants between 2005 and 2007, paying them 84,000 euros ($91,000).
Fillon, a devout Catholic who won the nomination of the Republicans party in November on a pledge to slash public spending, has flatly denied any wrongdoing.
He insists his family did real work and that he is the victim of a leftwing smear campaign.
The revelations have triggered a preliminary probe into possible misuse of public funds.
Le Monde newspaper on Monday quoted Fillon as telling investigators his children were paid from parliamentary funds to write a book and work on the 2007 presidential campaign of Nicolas Sarkozy.
Le Monde reported the probe could be widened as investigators begin examining Fillon's ties to the owner of a literary magazine who is also suspected of paying Penelope for a fictitious job.
The investigators are looking at whether the owner of the Revue des Deux Mondes magazine paid her around 5,000 euros a month between May 2012 and December 2013 in return for being recommended by Fillon for France's highest honour, the Grand Cross of the Legion of Honour, in 2010.
Fillon's lawyer Antonin Levy told the paper the reward "was in no way connected with Mrs Fillon's employment".
The scandal, which comes less than three months before the French begin voting in an election characterised by the kind of anti-establishment anger that fuelled Brexit and Donald Trump's rise to the White House, has cost Fillon crucial support.
Polls now show the former frontrunner now crashing out of the first round in April, leaving far-right candidate Marine Le Pen and rising star Emmanuel Macron, a centrist, to battle it out in May's runoff vote.
Fillon's listing poll numbers have created alarm in his camp, leading some members of his party to call for a replacement candidate.
Some have suggested 71-year-old Alain Juppe, the runner-up in the Republicans primary, should step forward.
On Monday, Juppe again ruled out taking over from Fillon.
"I've set out my position... No means no," Juppe told reporters in the southwest city of Bordeaux, where he is mayor.
Le Figaro quoted an unnamed Republicans lawmaker as saying that, despite his public renunciations, Juppe could be persuaded to step in if Fillon, 62, bowed out.
"Juppe will only go for it if there is consensus," the source was quoted as saying.