Former president Nicolas Sarkozy has been charged with taking financial advantage of France's richest woman, as part of a probe into illegal party funding that could shatter his hopes of a political comeback.
His lawyers said they would appeal against the decision to formally investigate Sarkozy over allegations he took advantage of elderly L'Oreal heiress Liliane Bettencourt when she was weakened by poor health.
He was unexpectedly summoned on Thursday to the Bordeaux offices of Jean-Michel Gentil, the judge in charge of the case, for face-to-face encounters with at least four former members of Bettencourt's staff.
The surprise confrontation came over claims he had accepted envelopes stuffed with cash from the world's richest woman to fund his 2007 election campaign.
Gentil was seeking to establish how many times Sarkozy had visited Bettencourt during his successful campaign.
Sarkozy, 58, has always maintained that he visited Bettencourt's residence only once during the campaign, to meet her late husband. Members of the multi-billionaire's staff have, however, contradicted his version of events.
Sarkozy's lawyer Thierry Herzog lambasted the decision to pursue his client as "legally incoherent and unfair". He said he would immediately initiate proceedings to have the charges dropped.
Back in November, Gentil and two other examining magistrates spent 12 hours questioning Sarkozy. They decided not to formally charge him then but to continue investigating the allegations against him.
France's legal system has nothing that exactly matches the charges or indictments brought in English, or US courts; but an investigating judge's decision to place someone under judicial investigation is the closest equivalent.
Bettencourt is now 90. Medical experts say her mental capacity began to deteriorate from the autumn of 2006.
The allegation is that Sarkozy obtained significant amounts of money from her, breaching electoral spending limits and taking advantage of a person weakened by ill health.
Bettencourt's former accountant, Claire Thibout, told police in 2010 she had handed envelopes filled with cash to Bettencourt's right-hand man, Patrice de Maistre, on the understanding it was to be passed on to Sarkozy's campaign treasurer, Eric Woerth. Woerth has already been charged in the affair.
Investigators suspect up to four million euros ($5.2 million) of Bettencourt's cash subsequently made its way into the coffers of Sarkozy's UMP party.
Sarkozy lost immunity from prosecution when he was defeated in the 2012 presidential election by Socialist Francois Hollande.
French judges have already successfully pursued Sarkozy's predecessor as president, Jacques Chirac, who was convicted in 2011 on corruption charges related to his time as mayor of Paris.
Chirac, excused from attending his trial because of ill health, received a two-year suspended prison term.
Since losing to Hollande, Sarkozy has concentrated on making money on the international conference circuit, but he has repeatedly hinted that he is considering another tilt at the presidency in 2017.
Earlier this month he told Valeurs Actuelles magazine that his sense of duty to his country could see him return to the political arena.
During his presidency he won international acclaim as the principal architect of the 2011 NATO campaign that ousted Libyan dictator Moamer Kadhafi.
But 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 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.
Anyone convicted of exploiting a person's weakened mental state can be punished by up to three years in jail, fined up to 375,000 euros ($484,000), and banned from holding public office for up to five years.
Sarkozy's troubles come just two days after France's budget minister Jerome Cahuzac resigned Tuesday, as prosecutors announced a full criminal inquiry into allegations that he had an undeclared bank account in Switzerland.
Edwy Plenel, co-founder of the Mediapart news website, which first ran the allegations against Cahuzac, told AFP: "This week shows the usefulness of independent journalism, which is very much in the public interest."
Mediapart has also taken a leading role covering the Bettencourt affair.