Former president Nicolas Sarkozy is due to be questioned about claims that France's richest woman illegally helped finance his 2007 electoral campaign, BBC News reported.
Sarkozy faces allegations of accepting thousands of euros from L'Oreal heiress Liliane Bettencourt when he ran for office five years ago.
Police raided his home and offices in July as part of an inquiry ordered by investigating judge Jean-Michel Gentil.
Sarkozy, who lost presidential immunity in May, denies all wrongdoing. He will appear before the judge in the southwestern city of Bordeaux.
Although he has stepped back from frontline politics since his defeat by Socialist challenger Francois Hollande, 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, BBC says.
Judge Gentil is leading the investigation into allegations that staff acting for Bettencourt gave 150,000 euros (£120,600) in cash to Sarkozy's aides during his successful 2007 campaign to become president.
Individual campaign contributions in France are limited to 4,600 euros ($5,800).
Bettencourt's former accountant, Claire Thibout, alleges 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 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.
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 Sarkozy and Woerth first surfaced in connection with a trial over the estimated 17bn euro fortune of Bettencourt, 87, whose father founded the L'Oreal cosmetics giant.
Both Woerth and Sarkozy deny any wrongdoing, as does Bettencourt.
