French centre-right presidential candidate Francois Fillon has said he will fight on, as he announced that a judge was placing him under formal investigation, according to BBC News.

For weeks, he has fought allegations that his wife was paid for years for work she did not do.

He has now been summoned to appear before the judge, Serge Tournaire, on 15 March. "It's a political assassination," Fillon complained.

A former prime minister during Nicolas Sarkozy's presidency, he was selected late last year in national primaries held by the centre-right Republicans that attracted some four million voters.

For a time he was the favourite in the race to succeed Francois Hollande as president, but then came the "fake jobs" allegations in satirical weekly Le Canard Enchaine.

His appearances have recently been accompanied by loud protests and he has accused the government of allowing the campaign to turn into "a climate of quasi civil war".