One of France's main investigative news programmes, Envoye Special, is set to air previously unseen footage on Thursday evening of Fillon's Welsh-born wife Penelope talking to a journalist in 2007.
Envoye Special presenter Elise Lucet said that "several interesting remarks" had been found in the footage, including that Penelope had never -- contrary to recent revelations -- acted as her husband's assistant.
Penelope Fillon's lawyer, Pierre Cornut-Gentille, insisted the remarks contained in an interview for the Sunday Telegraph newspaper had been "taken out of context."
The footage has not been broadcast or seen by other media, including AFP.
Fillon, a conservative, had been leading the election race until the Canard Enchaine newspaper reported that his wife earned 830,000 euros ($900,000) as a parliamentary assistant over more than a decade -- despite no-one recalling her ever working at the National Assembly.
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A poll on Wednesday showed Fillon would crash out in the first round of the election in April behind far-right leader Marine Le Pen and 39-year-old centrist Emmanuel Macron, who is rising fast in the polls.
The poll showed Macron easily defeating Le Pen in a May runoff.
On Thursday, Fillon ploughed on with his campaign, visiting a village on the Belgian border, where he refused to answer questions on "Penelopegate" as the French media have dubbed it.
The scandal has dismayed rightwing voters, who see the election as theirs to lose after five years of troubled Socialist rule.
"It's scandalous that his wife received such huge sums of money," Anne Serise-Dupuis, a 66-year-old psychiatrist said in the western city of Bordeaux. "He should withdraw. He has been disqualified," she said.
In the southern city of Marseille, former shipping company boss Henri Delannoy said he had voted for Fillon "as an alternative to (Nicolas) Sarkozy in November's Republicans primary.
"But it's going to end up with everyone voting for Macron," he said.
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Juppe as Plan B?
Fillon, 62, has flatly denied the accusations that he used public money to pay his previously low-profile wife for a fictitious job.
This week it emerged that he also had two of his children on the payroll, for an additional 84,000 euros.
Employing family members is not illegal in France -- providing they have real jobs -- and Fillon has painted the allegations as a plot by the ruling Socialists.
Investigators this week raided his parliamentary office and interviewed the couple as part of a preliminary probe into alleged misuse of public funds.
His wife's lawyer said Thursday that Penelope had handed over "all the details proving the existence of a real job".
The accusations are highly damaging for Fillon, who won the Republicans nomination in November as a sleaze-free reformer who would slash France's debt by cutting 500,000 civil servants' jobs.
Republicans MP Philippe Gosselin said Thursday that an open letter was being drawn up by some members of the party calling on 71-year-old Juppe, a moderate, to take over if Fillon decided to stand aside.
But a group of Republicans leaders said they were standing by their man.
"We give Francois Fillon our complete support because his commitment to France is vital," the group, which included former finance minister Francois Baroin, touted as a possible replacement for Fillon, wrote in the rightwing Le Figaro daily.
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Juppe, whom Fillon beat in the primary, has ruled out being a "plan B".
But a new poll showed almost seven out of 10 French voters wanted Fillon to quit the race, even if a majority of Republicans supporters still had faith in him.
Meanwhile, the surprise winner of the leftwing nomination for the election, Benoit Hamon, met President Francois Hollande as he searches for support within the fractured Socialist party to mount a credible challenge.
Hamon, a 49-year-old radical leftwinger with an ambitious proposal to pay a universal basic income in a world of dwindling work, will probably need to win over the votes of Communist-backed Jean-Luc Melenchon to stand any chance.
Hollande, who decided not to run for a second mandate after a trouble-plagued five years in power, has yet to publicly endorse the Socialist nominee.
