Fillon, who is battling allegations his wife earned nearly €800,000 from fake jobs as a parliamentary assistant, told the media on Monday that his wife's salary was “perfectly justified for the indispensable work she did for him”.
Blasting the attempted “political assassination” against him, conservative candidate Fillon admitted his wife had worked for him and for his friend and fellow MP Marc Joulard for around 15 years.
He lambasted the figures in the media for how much his wife was alleged to have earned, insisting she picked up an average of €3,677 per month.
That salary was totally justified for someone with a law degree, Fillon said, dismissing the idea that he should pay it back.
Addressing allegations that Penelope did not actually perform any duties to earn hundreds of thousands of euros, Fillon said: “No-one has the right to judge what a parliamentary assistant's job consists of, except the MP himself.”
He did however apologize to the French people, admitting that employing his wife and children was “a mistake”.
“I apologise to the French people,” he said, admitting it was an “error” that he regretted “deeply”. But he steadfastly refused to stand aside.
The one time favourite to be the next French president before the fake jobs scandal said everything he did was “legal” but accepted that morally, the French public could no longer tolerate MPs employing their family members as aides.
“What was acceptable yesterday is no longer acceptable today. Working with your own family is now a practice that has been rejected,” he said.
Asked why he did not admit his error when the scandal first broke last week Fillon says he was “destabilized”.
“It took me time to react and to realise the sky had fallen on me,” he said.
Fillon spoke to over 200 journalists in an eagerly awaited press conference that came as the latest polls show the fake jobs scandal has sent Fillon's chances of being the next president tumbling.
Fillon's presidential bid has been floundering since the Canard Enchainé newspaper claimed that his wife Penelope earned more than €800,000 ($860,000) over a decade as a parliamentary aide to her husband and an ally.
Polls now show the former frontrunner would now crash out of the first round of the election in April, beaten by both the National Front's Marine Le Pen and former minister turned independent candidate Emmanuel Macron.
British-born Penelope Fillon is suspected of having been paid without doing any of the normal work of a parliamentary aide. In the past, she has said she was never her husband's assistant.
Fillon also paid two of his children to act as assistants.
All the revelations against him have triggered an investigation into possible misuse of public funds.
Fillon, a devout Catholic who won the nomination of the Republicans party in November on a pledge to slash public spending, has vehemently denied any wrongdoing.
“I'm not perfect buy my life in politics speaks for me,” he said.
But his listing poll numbers have sown alarm in his camp, leading some members of his party to call for a backup candidate.
Some have suggested former prime minister Alain Juppé, the runner-up in the Republicans primary, should step forward.
But 71-year-old Juppé on Monday again ruled out being a surrogate candidate.
Fillon however has now made it clear he is going nowhere.
“Nothing will make me change my mind. I am the candidate in the presidential election,” he told reporters, before stressing: “I am the candidate to win.”
Fillon said he campaign will begin again in earnest from tomorrow, pointing out he had the backing of four million French people who had voted for him in November's primary.
Back then, when Fillon had just triumphed over Juppé the presidential was seen as his to lose, but after the damaging revelations he has everything to do to win it.