Jérôme Kerviel, 31, has been placed under official investigation but allowed to walk free on condition he remained in the country. He faces multiple charges of forgery, computer hacking and breach of trust, but the charge of attempted fraud was not pressed.
Prosecutors said that he had behaved "like a financial drug addict" as he bet wildly on stock markets. They said they would appeal against his release.
But in a series of counter accusations against Société Générale, France's second biggest bank, Mr Kerviel said he and other traders had regularly exceeded trading limits set for them by the bank.
Prosecutor Jean-Claude Marin said that the £75,000-a-year junior trader had not stolen money from the bank but had hoped to secure a higher salary and large bonus and to boost his reputation as an "exceptional" trader.
Mr Kerviel admitted concealing deals but told investigators he was a loyal employee who only wanted to raise the bank's profits.
Mr Kerviel claimed his strategy had been hugely successful, that he was almost £1 billion in profit in December and that he had been rewarded by the bank with a guarantee of a £200,000 bonus for 2007.
He claimed to have begun his activities in late 2005, whereas the bank has said they only stretched back a year.
SocGen faced further embarrassment after it was disclosed that the bank had been warned about Mr Kerviel's massive trading volumes in November by a derivatives exchange. He produced a fake document which threw his bosses off the scent, Mr Marin said.
Several shareholders also filed a complaint with the French financial markets watchdog yesterday after it transpired that a member of SocGen's board sold shares worth €86 million on Jan 9 - shortly before the scandal broke.
Mr Kerviel claims SocGen then brought the disastrous situation on itself by hastily selling his positions last week when they discovered the scale of his trades. At that time he held positions worth about £37 billion, more than the market value of the bank.
Mr Kerviel's version of events is very different from that given by SocGen head Daniel Bouton, who has compared the young trader to a lone "terrorist" in the bank's midst and a "great pretender".
A hunt for possible accomplices began with police focusing on calls and text messages Mr Kerviel sent and received on his mobile phone.
But Elisabeth Meyer, one of Mr Kerviel's lawyers, said: "It's a lynching. He has been thrown to the lions before being able to explain himself."The scandal has added to the deep distrust the French have of free market capitalism, at a time when President Nicolas Sarkozy is trying to push through relatively liberal economic reforms and encourage the French to embrace financial success.