Guéant's latest legal troubles concern a contract to provide opinion polls to the presidency signed with the Publifact company, made without calling for competing tenders.
It authorised the company to buy the results of polls on any subjects it wanted from any source it wanted for a fee of 10,000 euros per month.
A 2009 audit found that Publifact, which later became Publiopinion, delivered 250 polls, some of which had already been published in the media, to the Elysée presidential palace between 2007 and 2009 at a 65-70 per cent profit of 1.4 million euros.
Publifact bosses Patrick Buisson, who was also an adviser to the president, and Pierre Giacometti have already been charged with profiting from favouritism.
The Elysée was found to have spent 7.5 million euros on opinion polls during Sarkozy's second term of office, which ended in 2012.
Guéant is already appealing against a two-year suspended prison sentence for paying large cash bonuses to interior ministry personnel when Sarkozy was minister and he was chief of staff.
He has also been charged with forgery and money-laundering in relation to alleged Libyan financing for Sarkozy's 2007 election campaign.