Jerome Kerviel, a rogue trader at Societe Generale who circumvented the bank’s security controls and made a series of unauthorised transactions, has been jailed for three years and handed an unprecedented £4.3 billion fine. Kerviel was today found guilty of unauthorised computer use, forgery and breach of trust. At a court in Paris, he was also handed a two-year suspended sentence.

The trader will have to repay damages of €4.9 billion (£4.3 billion) that the bank said it lost through his trades. In court, ex-Societe Generale chief executive Daniel Bouton said the trades had been viewed by the company as a “catastrophe” and branded Kerviel a “financial terrorist”.

Kerviel will appeal the conviction. He has always maintained the bank knew of his trades and turned a blind eye to his actions because he was earning it money. He had been accused of spending €50 billion of the bank’s money without its knowledge.

Judge Dominique Pauthe told the court: “Kerviel knowingly went beyond his remit as a trader." Kerviel has been barred for life from trading.

Kerviel, who now earns around £2,000 a month working for a computer services firm, is not expected to ever be able to pay back much of the huge fine, even if he were to have a huge salary increase. On his current salary and with regular pay increases, it would take over 120,000 years to pay back, French media calculated.

Kerviel had previously worked in the bank's IT department, and so had in-depth knowledge of its systems and procedures. Among the tricks Kerviel used to hide his activities, according to an internal report at the bank, he used fake email messages to justify missing trades, and borrowed colleagues’ login credentials to conduct trades in their name.

Between July 2006 and September 2007, internal control systems at the bank raised 24 alerts when the value of Kerviel's trades exceeded authorised limits. At the time, the bank's risk monitoring unit put the anomalies down to recurrent problems with the way the trading software recorded operations.