Former French president Nicolas Sarkozy has been formally placed under investigation over claims that he illegally took donations from France's richest woman on the way to his 2007 election victory.

A judge yesterday filed preliminary charges in a campaign finance case, after Mr Sarkozy, 58, went through hours of questioning in a Bordeaux courthouse, according to the prosecutor's office.

The ex-president is accused of "abuse of someone in an impaired state" in the case involving L'Oreal cosmetics fortune heiress Liliane Bettencourt, who is now 90.

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 potentially could join his predecessor and former mentor, ex-President Jacques Chirac, who was convicted after office.

In a political financing scandal of his own, he became in 2011 the only former French leader since Second World War-era Nazi collaborator Marshal Philippe Petain to be charged or convicted of a crime.

The charges are unlikely to have any immediate political impact, as the conservative Mr Sarkozy said his political career was over and assumed a low profile after losing his re-election bid to Socialist Francois Hollande in May.

While some polls suggest Mr Sarkozy is the mainstream right's favoured candidate in the next presidential race, it is not until 2017.

However his troubles were likely to take the media spotlight off the political scandal that hit Mr Hollande's government this week, with the resignation of Budget Minister Jerome Cahuzac over allegations that he moved cash abroad to avoid paying French taxes.

The investigation in Bordeaux centres on the finances of Ms Bettencourt, who was once the focus of a long-running family feud over her fortune.

Ms Bettencourt, who was reported to suffer from Alzheimer's disease, has since been placed under legal protection.

Mr Sarkozy lost his legal immunity from prosecution when he failed to win re-election.

In November, he was given the status of a so-called "assisting witness" with the possibility of facing charges later on allegations of abusing someone in an impaired state, swindling and abuse of confidence.

After yesterday's questioning, a three-judge panel opted only to retain the first of those counts related to activity in February 2007 and throughout that year, according to the prosecutor's office.

It emphasised that the former president is still presumed innocent of any wrongdoing.

Investigating judge Jean-Michel Gentil was looking into conflicting accounts about how many times Mr Sarkozy visited the home of Ms Bettencourt in the run-up to his winning 2007 campaign for president, according to one lawyer.

Earlier in the probe, Ms Bettencourt's ex-accountant told police she gave 150,000 euro (£125,000) to the manager of Ms Bettencourt's fortune that was to be passed on to Sarkozy's campaign treasurer.

Mr Sarkozy's lawyer and spokeswoman did not return calls seeking comment about the decision.

But Thierry Mariani, an ally in Mr Sarkozy's conservative party, suggested the charges were politically motivated.

He claimed they were part of an effort to discredit Mr Sarkozy just as polls suggest he is still widely liked and show big disappointment in Mr Hollande - whose popularity has plunged just 10 months into his five-year term.

A lawyer for Mr Sarkozy condemned the investigation.

Thierry Herzog said today that the former president remains "combative" but still considers he "has been treated scandalously".

Mr Herzog's comments on French radio RTL came a day after a judge filed preliminary charges against Mr Sarkozy.

The charges carry a maximum penalty of three years in jail and a 375,000 euro fine.