On Wednesday night, Sarkozy appeared on television for the first time since leaving the Elysee Palace in 2012 to claim the justice system was being used as a "political instrument" against him and to lash out at the magistrates and France's Socialist government.

"In our country, the country of human rights and the right of law, there are things that are being organised ... everything is being done to give an image of me that is not the truth.

"To all those watching and listening, I want to say that I have never betrayed their confidence. I have never committed an act against republican principles or the law," he said.

Wearing a sombre suit and tie, the former president looked tanned and clean-shaven - recent photos showed him with fashionable stubble - and strongly proclaimed his innocence. He described the accusations as "grotesque" and accused the judges of being politically biased and determined to humiliate and destroy him.

Before losing the 2012 presidential elections, Sarkozy promised to disappear from public life if he lost, telling voters they would "not hear from" him again.

Nearly three years on, few on any part of the French political spectrum believe he does not intend to stand for a second term of office in 2017.

The current case was sparked by information that allegedly emerged after detectives tapped Sarkozy's mobile phone as part of a separate probe into alleged illegal political campaign donations.

After getting wind of the taps, Sarkozy reportedly took out another phone subscription under a pseudonym to speak to his lawyer, which detectives also bugged.

Investigators claim the secretly taped conversations suggest the ex-president and his lawyer were trying to obtain information from magistrates about the campaign donation case and, most damagingly, had allegedly offered one magistrate a high-powered job in Monaco if he passed on legal secrets. Thomas Guenole, a lecturer at the Institute of Political Science in Paris, said he believed only direct proof of illegal activity or a conviction would stop the former president making a comeback.

"Whenever there is a legal problem, there is the same legal defence from Sarkozy. It is: 1, this is a plot against me; 2, all this is not happening by random chance; and 3, the prosecution has nothing, the case against me is empty," Guenole said.

"Almost all the right-wing sympathisers agree with the idea that it's a conspiracy and he's being persecuted."

Guenole added: "There are only two things that can stop Sarkozy's return to politics - actual and absolute proof that he personally and directly did something illegal, or if he is brought to trial and convicted.

"Nothing else will work to stop the bulldozer. Nothing. But we talk about a political comeback when, in fact, Sarkozy never went away."

Sarkozy has been preparing for a return to the political frontline in September, when it is anticipated he will reclaim the leadership of the UMP, France's main centre-right political party.

His allies say his legal problems, which they ascribe to pressure from the ruling Socialists, will only make him all the more determined.

"The more they go after him, the more that gives him the will to fight. Wait and see - he will come back stronger than ever," said a close political ally.

Sarkozy has been in this situation before. Last year he was charged with taking advantage of France's richest woman when she was too frail to know what she was doing, in a case that centres on envelopes of cash allegedly being passed to UMP officials.

When the charge was finally dropped in October, his path back to power appeared to have been cleared.

Two years after he walked out of the Elysee Palace vowing never to return, polls made him one of the country's most popular politicians and easily the centre-right's best chance for reclaiming the presidency in 2017.

Nine months later, the outlook is not quite as rosy. The drip-drip negative impact of being constantly linked to corruption cases appears to have chipped away at his standing with voters on the right of the political spectrum.

"He has gone from 66 to 50 per cent support as the best presidential candidate," said Jean-Daniel Levy, of the Harris Interactive polling agency.

"It is true he is still at the top of the list, but (former prime minister) Alain Juppe is close on his heels now and is looking like someone who can both unite the right and attack the left."

After his humiliating 2012 defeat by Francois Hollande, Sarkozy largely shunned the limelight, concentrating instead on making money on the international conference circuit.

But he has been unable to resist the lure of a return to the cut and thrust of political life.

Sarkozy has always been something of an outsider in the staid world of French politics.

The son of a Hungarian aristocrat who arrived penniless in France, Nicolas Sarkozy de Nagy-Bosca burst onto the political scene as a town mayor at 28, an MP at 34 and minister at 38.

He won the presidency at only 52 and was initially seen as a much-needed breath of dynamism, making a splash on the international scene and wooing the corporate world.

Sarkozy broke a longstanding taboo by putting his private life on display, divorcing his second wife while in office and publicly wooing the supermodel and singer Carla Bruni.

He married Bruni in 2008 and they had a daughter, Giulia, a few months before the 2012 election.

But as France's economy struggled amid the wider eurozone economic crisis, Sarkozy's public image took a beating.

His so-called bling-bling style - the seeds of which were laid with a champagne-soaked election night party at a glitzy Champs Elysees restaurant - provoked outrage as job losses mounted.

Hollande, a mild-mannered Socialist party apparatchik, seemed the perfect antidote two years ago, but has since run into the same problems that Sarkozy encountered when trying to revive or reform the French economy.

With the next election there for the taking, Sarkozy will hope it is not out of his reach.