The report came a day after Fillon apologized to the public for having employed members of his family while still a member of parliament, a scandal that has derailed his campaign.
The weekly Le Canard Enchaine alleged that Fillon had approved hundreds of thousands of euros to be paid to his British-born wife Penelope for fraudulent work as a parliamentary assistant while two of his grown-up children had received salaries for work for which they were not qualified.
Significantly, Mrs Fillon was not at today's press conference, and has not uttered a single word in public since the scandal broke nearly two weeks ago.
The former prime minister, 62, said he had hired family members - as allowed in France - out of "trust" but recognised that such practises "create distrust nowadays". "It was an error, I profoundly regret it and I apologise to the French".
If he were forced to quit as the centre-right's nominee, it would be an unprecedented move in six decades of French politics.
However, French prosecutors have launched a preliminary investigation into the possible misuse of public funds to determine whether or not Penelope Fillon in fact did any work for her husband.
"Her salary was perfectly justified because her work was indispensable to my activities as an elected official", he said.
However this email was sent on January 30th before the emergence of the crucial video on February 2nd which contained Penelope's infamous "I've never been his assistant" quote that François Fillon said had been taken out of context.
The former presidential favorite would be eliminated from the first round, beaten by both the National Front leader Marine Le Pen and former economy minister and independent candidate Emmanuel Macron, according to an OpinionWay poll for French Radio Classique radio and Les Echos newspaper published Monday.
The Republican nominee's recovery plan comes in the face of falling in support in the polls since a January 24 report saying that his wife had worked as his parliamentary assistant for many years while showing little to no presence in parliament.
The first round of presidential elections in France is scheduled for April.
During the press conference, Fillon listed the company's key clients and said no Russian business had paid him nor the Russian government, saying that all conferences he gave in Russia were free.
He said his wife has worked for him in his constituency in the central Sarthe region, managing his mail and his diary as well as his local meetings.