Dominique de Villepin, the former French prime minister, will be questioned by judges over allegations that he was involved in a plot to smear Nicolas Sarkozy and ruin his presidential campaign.

Luc Brossollet, One of Villepin's lawyers, confirmed on Friday that Villepin was now formally under investigation.

Villepin will be asked "on the expected count of complicity in libelous allegations" and other counts related to faked documents.

Villepin denied any involvement in the scandal in which Sarkozy's name appeared on a faked list of accounts purportedly held at Luxembourg-based securities clearing house, Clearstream.