Sarkozy could face questioning as soon as mid-June because he will lose his immunity a month after his successor, Socialist Francois Hollande, is sworn in on May 15.

Sarkozy denies any wrongdoing but might have to face court proceedings. His predecessor Jacques Chirac was convicted last year and given a suspended sentence, demonstrating that French courts are willing to try former leaders.

The most high profile case involves a series of overlapping inquiries surrounding alleged illegal campaign financing by L'Oreal heiress Liliane Bettencourt, France's richest woman.

Magistrates are investigating claims that Bettencourt's staff handed over envelopes stuffed with cash to Sarkozy aides to finance his 2007 campaign, with her former book-keeper testifying to one 50,000 euro donation.

Under France's electoral code, individual election campaign contributions must not exceed 4,600 euros.

Sarkozy and his camp have also been accused of ordering an illegal police investigation to identify an official leaking information on the Bettencourt scandal to a journalist from the newspaper Le Monde.

Judges have charged both a prosecutor close to Sarkozy and the head of France's domestic intelligence agency, Bernard Squarcini, with having illegally obtained the journalist's mobile phone logs in 2010.

Another case is been the so-called "Karachi Affair", in which two close aides to Sarkozy have been charged by judges investigating alleged kickbacks on a Pakistani arms deal.

The case dates back to Sarkozy's time as budget minister, when it is alleged that he authorised the creation of a company to channel kickbacks into then prime minister Edouard Balladur's ultimately unsuccessful 1995 presidential campaign.

A more serious allegation relating to the same case is that a bomb in 2002 in Karachi, which killed 11 French engineers, was an act of revenge organised by Pakistani officials who were angry at the cancellation of bribes secretly promised to them.

So far no investigation has been launched accusations made by a French website last month, that former Libyan leader Moamer Kadhafi donated 50 million euros to Sarkozy's 2007 presidential campaign.

Sarkozy denounced the claims as "grotesque" and said he would sue French media website Mediapart over the reports.

The Socialists pounded Sarkozy over the scandals during the election campaign, with Segolene Royal, Sarkozy's Socialist opponent in the 2007 race, saying he was desperate to hold on to office if only to dodge prosecution.

Sarkozy has said he plans to retire from politics. 

Chirac was called before investigators less than two months after he left office in 2007, on charges of breach of trust and embezzlement between 1990 and 1995, when as mayor of Paris he employed ghost workers.

He was the first post-war French head of state to be convicted of criminal wrongdoing.