Judges have filed preliminary charges against the trader accused of causing billions of euro in losses at French bank Société Générale.
Jerome Kerviel (31) has been released from custody while the investigation continues.
But the Paris prosecutor's office said it was appealing yesterday's decision to free Mr Kerviel, who had been held since Saturday.
The preliminary charges are for "breach of trust," "forgery and using forgeries" and unauthorised computer activity. The lighter-than-expected charges have resulted from the bank and prosecutors' belief that Kerviel did not appear to have profited from his unauthorised dealings.
Mr Kerviel's lawyers have described him as a "modest boy" who got in over his head.
However, the charges, filed after questioning Mr Kerviel for 48 hours, could bring him up to three years in prison and hefty fines.
It has emerged that the trader begun his deception in late 2005. His speculation eventually triggered "a certain number of alerts" from the middle office, accounting or risk services, according to the prosecutor.
He said that Eurex, a derivatives exchange owned by Deutsche Boerse, had questioned Mr Kerviel's trading positions in November 2007, and that Mr Kerviel had been able to sidestep questions from his employer.
"Questioned by the bank, he produced a fake document to justify the risk cover," the prosector said.
SocGen has said it only found out about the uncovered position on January 18th.
Additional reporting: PA