France's top appeals court on Wednesday rules in a case of alleged illegal campaign financing against former president Nicolas Sarkozy -- his last chance to escape adding a second definitive conviction to his growing rap sheet.

The Court of Cassation in Paris will rule on whether a lower court was right to convict him of illegal financing in his failed 2012 re-election campaign.

If it upholds Sarkozy's conviction, he will serve a six-month term, possibly with an electronic bracelet.

Sarkozy, a one-term president from 2007-2012, has faced a series of legal challenges since leaving office.

Wednesday's hearing is his last chance to escape a second conviction after he was sent to jail last month in a separate case, related to an earlier election campaign.

In that trial, the 70-year-old was found guilty of allowing aides to seek to collect money for his 2007 presidential run from Libyan leader Moamer Kadhafi.

His 20 days behind bars made him the first post-war French leader to serve jail time before he was released on November 10 under judicial supervision pending an appeal in that case also.

In December last year, he exhausted his last legal recourse in a third case, after being convicted of trying to extract favours from a judge.

He served that sentence with an electronic ankle tag, which was removed in May after several months.

- Campaign funding accusations -

The case under review on Wednesday focuses on charges that Sarkozy's right-wing party worked with a public relations firm, Bygmalion, to hide the true cost of his 2012 re-election bid.

Prosecutors said Sarkozy spent nearly 43 million euros (almost $50 million) on his 2012 campaign, nearly double the permitted amount of 22.5 million euros.

Unlike his co-defendants, he was not implicated in the double-billing system allegedly used to cover costs but was held accountable as the beneficiary of illegal campaign financing in his capacity as a candidate.

Sarkozy has denied "any criminal responsibility" in the case, denouncing the allegations as "lies".

A lower appeals court in February last year confirmed his conviction in the case, but the Court of Cassation could order a retrial if the appeal is found to be valid.

Despite his legal problems, the ex-leader remains an influential figure on the right.

He was received by President Emmanuel Macron before entering prison and Justice Minister Gerald Darmanin, a former protege, sparked criticism by meeting Sarkozy inside Paris's La Sante jail.

A court later barred Sarkozy from seeing the minister and other officials as part of restrictions tied to his release from prison.

A fortnight after his release, the ex-leader announced he would publish a book next month about his experience of serving three weeks in jail.