Mr Fillon has been embroiled in the so-called Penelopegate after claims he gave his wife Penelope a bogus job paid for with taxpayer cash.
Mrs Fillon, originally from Llanover, Wales, has been indicted over claims she received public money for work she did not actually carry out from 1986 onwards.
But new allegations suggest Mrs Fillon started to receive money from her husband’s office in 1982, when she was listed as a “parliamentary assistant”.
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Mr Fillon, who has seen his popularity slide from first to being one point away from joint fourth in the election race standings, originally said his wife was employed by him from 1986.
According to MediaPart Live, Mrs Fillon “has benefited from public money from the first parliamentary term of her husband, in 1982, through study contracts or assignments commissioned by him”.
The 63-year-old Le Mans-born politician was originally indicted on March 14 relating to claims of embezzlement of public funds, complicity and concealment of abuse of social assets.
Francois Fillon and his wife, Penelope, are embroiled in a financial scandal ahead of the election
His wife was then indicted on March 28 amid allegations of complicity in the concealment and embezzlement of public funds.
Initially, Mrs Fillon was allegedly paid about £450,000 over eight years by her husband. Then publication Le Canard Enchaîné revealed she had potentially been paid a further £260,000 than previously reported.
The former French Prime Minister also stands accused of paying his five children almost £72,000 for being “assistants”, some of them relating to legal assistance despite not yet being qualified lawyers.
It is alleged Fillon paid his wife in taxpayer money from 1982
But the lawyer for Mr Fillon and his family, Antonin Levy, finds it bizarre the debate over what year Mrs Fillon was employed is rearing its head now.
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He claims the investigators have been aware of the dates because they have had detailed documents for months and simply chose not to make them public.
Mr Levy said: “The right question to ask is why the national financial prosecutor, who had known for weeks, did not mention it, and why this information comes out 14 days before the first round of voting.”
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Mr Fillon came out and said “it was a mistake and I apologise to the French people”, but he did go on to defend his wife’s salary.
The French go to the polls for the first time on April 23 and if there is no majority winner then a run off between the top two candidates will take place on May 7.
Front National’s leader Marie Le Pen is consistently topping polls to win the first round of voting, but losing to En Marche! Leader Emmanuel Macron in the second round of voting.