Kerviel, 31, has been charged with falsifying documents and unauthorised computer access but was released on Monday pending the outcome of the investigation.

The junior trader has told investigators during questioning over the weekend that his bosses must have been aware of his dealings because of the profits he was generating.

Kerviel had been an employee at Societe Generale since 2000 when he joined the investment banking department, first as part of the support staff in the middle and back offices.

He moved up to the front office in 2005 and it is there that he began trading futures on European share indices, effectively betting on the future direction of the stock markets.

Societe Generale said that the trader had held positions worth about 50 billion euros when he was caught -- a figure well in excess of the bank's market value of 35.9 billion euros.