The row over a corruption probe against Nicolas Sarkozy has escalated after the judge who charged France's former president received a bullet and a death threat in the post.

Former French president Nicolas Sarkozy has been formally placed under investigation over claims that he illegally took donations from France’s richest woman on the way to his 2007 election victory.

Nicolas Sarkozy was questioned by magistrates trying to establish whether he received illegal campaign funds from France’s richest woman when he ran for president in 2007.

French investigators have searched the home and offices of former president Nicolas Sarkozy as part of a probe into suspected illegal financing of his 2007 presidential campaign, an official said.