Who really masterminded France's crime of the century?

For 34 years, the legend of Albert Spaggiari as the daring mastermind behind France's crime of the 20th century has blossomed.

From the raid on a bank via an underground tunnel to the daring escape on the back of a motorbike, his story has spawned films, books and theories of CIA involvement.

But now the legacy of one of the anti-heroes of modern French folklore – who taunted the authorities for their failure to catch him by sending photos of himself dressed as Father Christmas – is under threat. After years of stewing in silence, the man who claims to have been the real brains behind the operation has finally spoken out. Far from being the mastermind, Spaggiari was a mere informer – and a bad one at that.

"He did a useful job, but even that he didn't completely accomplish," the alleged mastermind, known only as "Amigo", told The Independent. "He didn't even know if the bank had an alarm. He didn't know the plan of the vaults, or how to get out."

In 1976, a gang tunnelled through the sewers and stole 50m francs (the equivalent of £24m today) from the vaults of Société Générale in Nice. They left behind empty bottles and a message on the wall: "Without hatred, without violence and without weapons." Even the mayor of Nice reluctantly said "hats off" for a heist so flawlessly executed.

Spaggiari was caught several months later, but escaped during his trial, apparently distracting the judge with a piece of evidence written in code and jumping out of the judge's window to meet an accomplice who was waiting on a motorbike. But even this is a romantic embellishment, according to Amigo, whose book on the raid was published this week. It was pure chance that one of his friends was hanging around in the road below on a motorbike.

Spaggiari spent the rest of his life in hiding, occasionally giving television interviews. Before his death in 1989, he published a memoir of the robbery and encouraged theories, films and books linking him to the government, Italian political movements and the CIA.

Amigo, who is still keeping a low profile, says he was bored of hearing nonsense about the case and so he decided to set the record straight for his friends. "Spaggiari just wanted to create a personality. How could an obscure photographer become the head of a gang?" he said. "He had to elaborate."

According to Amigo's version, Spaggiari did not even help with the digging. And he dismissed Spaggiari's claim that the message on the wall was a political statement. Amigo says he wrote it, having thought of it less than an hour beforehand. It was only designed to demonstrate the contrast between his gang and the kind of criminals who defecated at the scenes of their crimes.

In his book The Truth About the Nice Heist, Amigo introduces his band of tunnellers: Corsican, Fat, Baby Donkey, Crazy and Comb-over. There's also Small, whose footprints were so tiny the police had always believed there was a woman in there. Spaggiari obliged them, inventing in his version a prostitute called Mireille. Nonsense, says Amigo. The tunnel, 8m long, 80cm wide and 1.3m high, was dug completely by hand. Tools would have made too much noise. Though Amigo writes fondly of Spaggiari, he says he feels obliged to point out he was not a true enough crook to get his hands dirty.

Amigo – who has served time but never been arrested for the Nice bank job – has refused to reveal his identity, but said he now lives in Marseille, has a big family and has been an honest businessman for at least 15 years. His publishers said they made inquiries into his background to establish his credibility. They claim a police source identified "Amigo" as one of the suspected perpetrators but there was too little evidence to charge him.

Amigo is not proud of being the mastermind of the celebrated crime. "All the pleasures that come with the life of a crook do not make up for the heavy sacrifices," he said.