Mr Sarkozy faces accusations that his party falsified accounts in order to hide 18m euros (£15m; $20m) of campaign spending in 2012. Mr Sarkozy has repeatedly denied that he was aware of the overspending.
He lost the 2012 race to Francois Hollande, and failed in his bid to run again in this year’s upcoming presidential election. The case is known as the Bygmalion scandal.
It centres on claims that Mr Sarkozy’s party, then known as the UMP, connived with a friendly PR company to hide the true cost of his 2012 presidential election campaign.
France sets limits on campaign spending, and it is alleged the firm Bygmalion invoiced Mr Sarkozy’s party rather than the campaign, allowing the UMP to exceed the limit.