Former French president Nicolas Sarkozy has been charged with taking financial advantage of France's richest woman, in allegations connected with 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.

Graffiti on a campaign poster in Paris for Nicolas Sarkozy during the 2012 French presidential election. The former president has been charged with taking financial advantage of France's richest woman Liliane Bettencourt, as part of a probe into illegal party funding. [Photo/Agencies]

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 was 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. However, that version of events has been contradicted by members of the multi-billionaire's staff.

Sarkozy's lawyer Thierry Herzog on Friday 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 magistrates spent 12 hours interrogating Sarkozy. They decided not to formally charge him then but to continue investigating the allegations.

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 and 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, 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 $5.2 million of Bettencourt's cash subsequently made its way into the coffers of Sarkozy's UMP party.

The charges carry a maximum penalty of three years imprisonment and a $484,000 fine.

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. He was convicted in 2011 on corruption charges related to his time as mayor of Paris.

Chirac, who was excused from attending his trial because of ill health, was given a two-year suspended prison term.

AFP-AP