Investigators are looking at whether Fillon and his wife Penelope forged documents to try to justify the around €700,000 ($757,000) she earned for a suspected fake job as a parliamentary assistant, the source said.
Fillon, 63, has already been charged with misuse of public funds but has refused to bow out of the running for president.
Penelope Fillon's lawyer Pierre Cornut-Gentille rejected the new allegation, telling AFP: “There is not the slightest bit of forgery in this case.”
He denounced what he called a violation of confidentiality during an ongoing investigation. “We won't defend ourselves before facing the court,” he said.
Francois Fillon, who served as prime minster under ex-president Nicolas Sarkozy, is a canny political veteran who has spent a lifetime preparing for his shot at the Elysee Palace.
The clear frontrunner at the start of the year, he has been embroiled in scandal after scandal since January when he admitted to paying his wife and children hundreds of thousands of euros from public funds to act as his parliamentary assistants.
They are suspected of having done little or no work in return, leading Fillon to be charged on March 14th with misusing public money.
The 63-year-old devout Catholic wants to slash state spending and cut 500,000 public sector jobs over the five-year presidential term. Polls currently show him running in third.