Preliminary charges have been filed against Nicolas Sarkozy over allegations he took financial advantage of an elderly L'Oreal heiress.
The move means the former French president is under formal investigation, accused of accepting envelopes stuffed with cash from Liliane Bettencourt to illegally finance his 2007 election campaign.
The 90-year-old was declared in a state of dementia in 2006 and was placed under the guardianship of her family in 2011.
The preliminary charges were issued after Mr Sarkozy was questioned for several hours in a Bordeaux courthouse, according to the prosecutor's office.
A statement said he had been placed under investigation "for taking advantage of a vulnerable person in February 2007 and during 2007 to the detriment of Liliane Bettencourt".
Under French law, preliminary charges mean the investigating magistrate has reason to believe wrongdoing was committed, but allows more time to investigate. The charges may later be dropped or could lead to a trial.
Mr Sarkozy's lawyer Thierry Herzog criticised the decision as "legally incoherent and unfair".
Earlier, the former president was unexpectedly summoned for a face-to-face encounter with Ms Bettencourt's ex-butler Pascal Bonnefoy over the claims.
Jean-Michel Gentil, the judge in charge of the case, was hoping the confrontation would allow him to establish how many times Mr Sarkozy visited Ms Bettencourt during his successful campaign.
The 58-year-old has always maintained he visited her home only once, contrary to testimony from several members of the multi-billionaire's staff.
So far, 17 people have been charged in the case.
In November, magistrates decided not to formally charge Mr Sarkozy after a 12-hour interrogation but continued investigating the allegations against him.
Ms Bettencourt's former accountant told police in 2010 that she had handed envelopes filled with cash to the heiress' right-hand man, Patrice de Maistre, on the understanding it was to be passed on to Mr Sarkozy's campaign treasurer, Eric Woerth.
Mr Sarkozy lost his immunity from prosecution when he was defeated in the 2012 presidential election by Socialist Francois Hollande.
Since losing office he has had to contend with a string of allegations relating to his five years in power and various electoral campaigns he has been involved in.
As well as the Bettencourt case, he faces probes into alleged cronyism in the awarding of contracts for opinion polls, an illegal police investigation into journalists and alleged kickbacks on a Pakistani arms deal used to finance the right in 1995, when Mr Sarkozy was budget minister.
He has always denied any wrongdoing and remains popular with right-wing activists despite being regarded as a divisive figure among the swing voters who tend to decide French elections.