France's centre-right presidential candidate Francois Fillon has claimed allegations that he paid his wife for jobs she did not do were "carefully prepared" to destroy him.

The 62-year-old had a significant lead in the polls for April's elections, until the satirical weekly Le Canard Enchaine accused him of paying Penelope Fillon hundreds of thousands of Euros for jobs she may have not done.

Investigators are looking into the claim, however Fillon insists his innocence and claimed it is a ploy to give far-right Front National leader Marine Le Pen the upper hand.

"These attacks do not come out of nowhere," Fillon said at a political rally in northern France on Thursday (2 February), according to Le Monde.

"They were carefully prepared, simmered in the back kitchens of offices we will one day discover."

His comments came as investigators expanded the scope of the inquiry to include his two children, who were allegedly paid as legislative aides while Fillon was a minister.

In France, it is legal and commonplace for MPs to hire family members as long as they are genuinely employed.

Whether that was the case with Fillon has come into question. A 2007 interview of Penelope Fillon – where she said she did not work for her husband – has recently been re-circulated. It has also been revealed that Fillon's two children were not qualified as lawyers when they allegedly carried out the work.

Fillon claimed he was the centre of an attack due to his policies, which are widely considered to be conservative, but to the left of Le Pen.

"If I were a bland candidate, this outcry would not take place," Fillon said. "If my project was blurry, the fury would not be there."

He tried to quell the sense of betrayal some of his supporters felt, assuring them the allegations would pass without being proven.

Fillon said: "I understand that the charges disturb some of you, because of their pounding, because of the repetition cleverly presented as an accumulation of evidence ... But lies, even if they repeated a hundred times, will never be able to be a half-truth."

In a unique move, the establishment candidate has said he will withdraw from the race if the preliminary inquiry becomes a formal one. No French presidential candidate has ever dropped out of the race at this late a stage.