France's richest woman has received a bullet and a death threat in the post, say lawyers.
Jean-Michel Gentil, the most prominent of three judges investigating the case, received the threatening letter together with blank cartridges on Wednesday, the magistrate's union SM said in a statement.
One of Gentil's colleagues said the letter, which arrived at the judge's Bordeaux offices, contained threats against other magistrates. Police had been called.
The SM denounced what it called "insulting statements" made by some of Sarkozy's political allies, which it said were designed to undermine the judiciary.
It noted too that Sarkozy's own lawyer, Thierry Herzog, had questioned Gentil's impartiality.
The SM said a number of its members were targeted in the letter.
Sarkozy's lawyers are attempting to overturn last week's decision by three magistrates to charge him in a case that threatens to destroy his hopes of a political comeback.
He has repeatedly denied claims he accepted cash-stuffed envelopes from the world's richest woman Liliane Bettencourt to fund his successful 2007 campaign.
Medical experts say the mental faculties of the L'Oreal heiress began to deteriorate in 2006.
Sarkozy's lawyer questioned Gentil's impartiality in an interview with newspaper Journal du Dimanche.
Gentil last June put his name to a column signed by dozens of legal professionals in the leading newspaper Le Monde, accusing Sarkozy and his predecessor Jacques Chirac of "wishing to protect the corrupt", he pointed out.
Herzog said that five days after the column, Gentil ordered police to search Sarkozy's home, office and his secretary's house.
A number of Sarkozy's political supporters have denounced the judge.
On Monday, Sarkozy used his Facebook page to insist he had not taken advantage of Bettencourt.
Sarkozy had, in recent weeks, hinted that he was considering a return to the frontline of French politics.
Last week's decision by the judges to put him under formal investigation dealt a blow to those hopes.
Sarkozy could face up to three years in jail, a fine of 375,000 euros ($A462,221.13), and a five-year ban from public office if convicted.
