Jérôme Kerviel, France's rogue trader, still believes that he could have made an astronomic profit from his €50 billion of illicit market bets if his bosses at the Société Générale bank had not sacked him, he has told police.

Mr Kerviel is also convinced that his superiors let him engage in his monumental gambling because he was successful at it for 18 months. A transcript of his interview with police was leaked to the media today.

"I remain persuaded that they knew about my positions," said Mr Kerviel, pointing out that his supervisors had ignored repeated warnings about him from market operators. "I was generating cash, so the signals were not very worrying for them. As long as we win and no-one sees and everything is taken care of, they say nothing."

The confession of the 31-year-old Breton appeared as the board of SocGen, France's second bank, rejected calls from President Sarkozy and the political world to dismiss Daniel Bouton, its executive Chairman. It created a committee to find out why its managers were unable to prevent history's biggest trading scandal.

In his two-day session with police, Mr Kerviel described his pleasure at hitting his first €500,000 jackpot in mid 2005 after he was promoted from the lowly back office to become a trader. With his provincial university degree and humble background, the hairdresser's son was bitter at being treated as inferior to the other "golden boys". He was determined to prove himself by "making money for the bank alone and in no way enriching myself personally."

He devised a system for covering his tracks when he took out big unauthorised positions on future stock market movements. He scored with a bet that shares in Allianz, the German insurance group, would fall. They did so, shortly after when terrorist bombs exploded in London in July 2005.

Mr Kerviel, who is free on bail and faces a possible maximum five-year sentence on forgery and other charges, gradually raised his stakes, faking e-mails and faxes to cover unhedged trades. By last December he was both thrilled and worried to find that he had made an off-the-books profit of €1.4 billion. He was only able to claim credit for €55 million and earn the bonus that he yearned for.

"At that stage, I was out of my depth and I did not know how to report to the bank undeclared cash worth €1.4 billion," he said. "No-one had ever reached that figure. ... I was happy, proud of myself.. so I decided not to tell the bank and hide the money with a fake operation in the other direction."

Everything went well until late last Friday when his €50 billion bet on rising markets turned sour. That night his bosses were alerted and by Monday he was sacked as the bank rushed to sell off his position, racking up €5billion of losses. “They should have waited,” said the broker. "I remain convinced that in the next three months the market will bounce back."

Mr Kerviel, who has become a folk hero for much of France, said that his secret trading left so many traces that his bank could not have been in the dark. "The simple fact that I did not take any holidays in 2007 should have alerted the management. That is one of the primary rules for the internal controls." The replacement of traders on holiday enables the bank to check on their positions.

Asked whether he did not fear being discovered and sacked, Mr Kerviel said: "I was in a spiral which completely masked that question. And then it was too late." He was not calm about risking billions, he admitted. "I am aware that I have taken a very big position but I am deeply convinced that my analysis is well-founded."

Prosecutors and SocGen, renamed "ShockGen" in the business, depict Mr Kerviel as "psychologically fragile". Mr Bouton and his staff say that the trader outfooled the world's most sophisticated control system. His lawyers say that he is the victim of character assassination and a "media lynching" by his embarrassed employers.

Times Recommends

• Lies and cover-up at MG Rover exposed by report
• High oil prices push manufacturing costs up
• Primark founder steps down as chief executive