In the French justice system, being placed under investigation does not confirm wrongdoing but means investigators have serious grounds for pursuing the matter.

If charged and found guilty, Mr Fillon faces a 10-year prison term and a million-euro fine.

In a declaration to judges leaked to the French press, Mr Fillon insisted: "Yes, I did employ my wife and the reality of her work is undeniable. This reality has been confirmed in detail by several people who have worked by her side over many years."

From handling mail and his diary in liaison with his secretary to attending local events, he said: "For all these years, my wife has worked by my side and that of my replacement to ensure perfect continuity between him and me."

Once it had been established that his wife had done any work for him, Mr Fillon added: "It is not up to the judicial authorities to assess the quality or content of that work."

"I am not asking for exemption or favour but simply the respect of the law."

Although presumed innocent, the legal development is particularly damaging for Mr Fillon as he has previously depicted himself as a morally spotless white knight of French politics, remarking in primaries for his Les Républicains party in November: "Who could imagine Charles de Gaulle placed under formal investigation?"