Jérôme Kerviel, the rogue trader who plunged Société Générale into crisis when he lost €4.9 billion (£3.7 billion), went undetected because controllers failed to follow up repeated alerts about his behaviour, according to a report published last night.

The investigation committee set up by SocGen to look into the biggest rogue trader scandal in history added that Mr Kerviel had been given a €300,000 bonus last year despite growing signs that his dealings were unauthorised.

The report broadly backed the French bank’s claim that Mr Kerviel had spun a web of lies around his unauthorised positions.

The committee, chaired by Jean-Martin Folz, the former chairman of Peugeot Citroën, the carmaker, also agreed with Daniel Bouton, SocGen’s chairman, that the trader appeared to have acted alone.

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However, Mr Folz also pointed to serious lapses in the bank in a criticism that could add to pressure for Mr Bouton to resign.

Although the report exonerates executives from direct responsibility for the fraud, it suggests that personnel did nothing more than go through the motions in the search for deception. It will add to claims that Mr Bouton was keen to avoid treading on his traders’ toes in the hunt for profits.

Mr Folz said that operators charged with controlling traders had failed to dig out the truth while Mr Kerviel placed wild bets on European futures markets. The failure to detect the fraud could be explained “by the fact that operators did not systematically deepen their verifications beyond what is required by the procedures in force”, Mr Folz said.

He added: “Operators did not . . . have the reaction to inform their superiors or those in the front office of the appearance of anomalies even of very high sums if that did not figure precisely in the procedures.”

Mr Folz also said that the absence of controls on modified or cancelled transactions – a practice used by Mr Kerviel – enabled the trader to get away with his lies. There were dozens of alerts about Mr Kerviel in 2007. Controllers questioned him each time, but failed to check whether he was telling the truth or to report suspicions to managers, the report said.

Mr Bouton will draw comfort from the report’s conclusion that Mr Kerviel appeared to have acted alone.

“At this stage of the investigations, there is no evidence of embezzlement or internal or external complicity [i.e. the existence of a third party who knowingly assisted the fraudster to conceal his positions],” the report said.

It added: “Investigations are continuing . . . to cover a wider area than the activities of the author of the fraud.”

The committee said that Mr Kerviel had obtained a €60,000 bonus for 2006. He asked for a €600,000 bonus for 2007 and got €300,000.

SocGen is today to announce a huge fall in profits to an expected €947 million. BNP Paribas, its French rival, yesterday unveiled a 7 per cent increase in its annual net profits to €7.8 billion. Fourth-quarter net profits fell by 42 per cent to €1 billion after a €897 million writedown for provisions and depreciations.

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