Nicolas Sarkozy is suspected of accepting thousands of euros from L’Oreal heiress Liliane Bettencourt, France’s richest woman.
Police raided his home and offices in July as part of an inquiry ordered by investigating judge Jean-Michel Gentil.
Nicolas Sarkozy, who lost presidential immunity in May, denies all wrongdoing.
Although he has stepped back from frontline politics since his defeat by Socialist challenger Francois Hollande, Nicolas Sarkozy is rumored still to harbor ambitions of running for re-election in 2017.
The outcome of the investigation could determine whether the former president will make his widely anticipated return.
Judge Jean-Michel Gentil is leading the investigation into allegations that staff acting for Lilliane Bettencourt gave 150,000 euros in cash to Nicolas Sarkozy’s aides during his successful 2007 campaign to become president.
Individual campaign contributions in France are limited to 4,600 euros.
Liliane Bettencourt’s former accountant, Claire Thibout, alleges Nicolas Sarkozy’s campaign treasurer at the time – Eric Woerth, who later became budget minister – collected the cash in person.
She also revealed in a leaked police interview that Nicolas Sarkozy, while mayor of Neuilly from 1983 to 2002, paid “regular” visits to the Bettencourt house.
The former president has dismissed as mere gossip claims that he took envelopes stuffed with cash.
Eric Woerth, who was forced to resign as UMP party treasurer in July as a result of the scandal, is already under formal investigation over the 150,000 euro payment allegations.
The allegations surrounding Nicolas Sarkozy and Eric Woerth first surfaced in connection with a trial over the estimated 17 billion euro fortune of Liliane Bettencourt, 87, whose father founded the L’Oreal cosmetics giant.
Both Eric Woerth and Nicolas Sarkozy deny any wrongdoing, as does Liliane Bettencourt.
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