Bordeaux - Former French president Nicolas Sarkozy was on Thursday charged with taking advantage of the elderly L'Oreal heiress Liliane Bettencourt, his lawyer told AFP, adding that he would immediately appeal.

The move came after Sarkozy was unexpectedly summoned for a face-to-face encounter with staff members of Bettencourt over claims he accepted envelopes stuffed with cash from the world's richest woman to illegally finance his 2007 election campaign.

Sarkozy's lawyer Thierry Herzog lambasted the decision as "legally incoherent and unfair".

He said he would immediately appeal and ask that the arraignment be dropped.

Overall 17 people have so far been charged in the case.

Judicial officials earlier told AFP Sarkozy had been summoned for an encounter with Bettencourt's former butler Pascal Bonnefoy.

Jean-Michel Gentil, the Bordeaux-based judge in charge of the case, was hoping the confrontation would allow him to establish how many times Sarkozy visited Bettencourt during his successful campaign.

Sarkozy, 58, has always maintained that he visited Bettencourt's residence only once during the campaign, contrary to testimony from several members of the multi-billionaire's staff.

Gentil and two other examining magistrates spent 12 hours interrogating Sarkozy in November. They decided not to formally charge him but to continue investigating the allegations against him.

Bettencourt is now 90 and has been in poor health since 2006. Sarkozy, it is alleged, obtained significant amounts of money from her, simultaneously breaching electoral spending limits and taking advantage of a person weakened by ill health.

Bettencourt's former accountant, Claire Thibout, told police in 2010 that 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.

Investigators suspect up to four million euros (US$5.2 million) of Bettencourt's cash subsequently made its way into Sarkozy's party coffers.

Sarkozy lost his immunity from prosecution when he was defeated in the 2012 presidential election by Socialist Francois Hollande.

French judges demonstrated their readiness to go after former leaders with their successful pursuit of Sarkozy's predecessor as president, Jacques Chirac, who was convicted in 2011 on corruption charges related to his time as mayor of Paris.

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.

Married to former supermodel Carla Bruni, the right-wing leader won international acclaim as the principal architect of the 2011 NATO campaign that ousted Libyan dictator Muammar Gaddafi.

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.