But Fillon's image of honesty was a key factor in him securing the Republicans' nomination, said Jean-Daniel Levy, of Harris Interactive pollsters, and he would run into trouble if voters now began doubting his character.
Le Canard Enchaine, which has lifted the lid on political shenanigans for decades, reported that Penelope Fillon had been paid 600,000 euros ($645,000) for many years of employment as a parliamentary assistant to him and later to his replacement as a National Assembly lawmaker, and for work at a cultural journal.
It said that its research had showed there was no evidence she had ever really worked.
Fillon's public relations team have emphasised there is nothing illegal about her working for her husband in the National Assembly.
They said her apparent lack of presence in the work-place by saying she preferred to work "in the background", in keeping with her self-effacing style.
Fillon and his wife, who is from Wales, were married in 1980 and have five children. Last October, she told a newspaper, Le Bien public: "Up to now, I have never been involved in the political life of my husband."
The image conveyed by glossy magazines and television shows is of a woman leading a country life and keeping home for her family in their 12th century chateau near Le Mans, west of Paris.