French presidential candidate Francois Fillon has vowed to keep campaigning despite being summoned by magistrates investigating payments made to his wife.

"I won't give in, I won't surrender, I won't pull out, I'll fight to the end," Fillon said in a speech in which he called the investigation "a political assassination."

Fillon must appear before the court on March 15. 

The conservative candidate has been battered for weeks by allegations that he paid his wife Penelope hundreds of thousands of euros of public money to be his parliamentary assistant, but that she actually did very little work. He denies any wrongdoing and says it was a proper job.

Alan Juppe, another former prime minister, has previously ruled out stepping in as the presidential candidate if Fillon, at one point the favourite, was forced to quit the race.

The investigation of Fillon and his wife has unnerved investors who fear Fillon's campaign woes have handed the anti-euro, anti-immigration Marine Le Pen of the National Front a higher chance of winning the presidency. Polls suggest, however, that she would lose in the second round to centrist Emmanuel Macron.