Former French president Nicolas Sarkozy has been placed under formal investigation over claims his 2007 election campaign received illegal donations from France's richest woman.

He is being investigated for "abuse of weakness" involving elderly L'Oreal heiress Liliane Bettencourt, the public prosecutor said.

Under French law, a formal investigation is the final step before a suspect is accused of a crime.

Mr Sarkozy, who only this month hinted he could make a political comeback, has repeatedly denied taking campaign funds from Ms Bettencourt.

He was questioned in November by judges, but they opted not to open a full-blown inquiry at that stage.

Initial suspicions over funding were fuelled three years ago.

A woman who worked as an accountant for the mentally frail heiress alleged that a large cash withdrawal was earmarked for Mr Sarkozy's presidential election campaign.

Ms Bettencourt, 90, was declared in a state of dementia in 2006 and placed under the guardianship of her family in 2011.

Her family has long had close connections with the UMP party of Mr Sarkozy, who lost presidential immunity when he left office.

Lawyers also are demanding that Mr Sarkozy explain himself in two other cases.

One is about the terms of a submarine sale to Pakistan.

The other case concerns lavish spending on opinion polls by his office when he was president.

Mr Sarkozy, who lost last year's election to Socialist Francois Hollande, faced members of Ms Bettencourt’s staff at yesterday’s hearing.

French TV channel BFM quoted Mr Sarkozy's lawyer Thierry Herzog as saying the decision was "incoherent and unjust" and he would appeal.

If found guilty, Mr Sarkozy, 57, faces a maximum three-year jail sentence and a hefty fine.

But the damage to his political career could be irreversible.

He gave the strongest hint yet that he might make a comeback bid earlier this month.

He told a magazine that a sense of duty to fix the economy might oblige him to run in 2017.

His remarks in Valeurs Actuelles increased speculation he could return to politics - talk that has not abated since he was ousted by Mr Hollande.

A poll published on 18 March showed Mr Sarkozy was the overwhelming favourite among conservative voters to run for president.

A survey the same week said he also was now more popular than Mr Hollande among the French, for the first time since November 2011.