Bosses at French banking giant Societe Generale were aware of the activities of "rogue trader" Jerome Kerviel, a top detective working on the case reportedly told an investigating judge, according to Mediapart.

The French investigative news website quoted Nathalie Le Roy as telling judge Roger Le Loire she was "certain" that Kerviel's superiors "could not have been unaware" he was taking wildly risky bets on derivatives.

Kerviel nearly brought Societe Generale to its knees in 2008 with losses of nearly five billion euros ($5.7 billion) from unwinding his trades of up to 50 billion euros ($57 billion).

"From different hearings and different documents that I've seen, I had the feeling, then I was certain, that Jerome Kerviel's bosses could not be unaware of the positions he was taking," Mediapart cited Le Roy as saying during her hearing.

She cited interviews she herself had carried out with an employee in the operational risk department of Societe Generale, who told her that "Jerome Kerviel's activities were known."

Societe Generale said in a statement it was "surprised" by the report.

"The case surrounding the fraudulent activities of Jerome Kerviel go back now more than seven years and there have been several court decisions which have always shown the sole criminal responsibility of Jerome Kerviel," the bank said.

"Societe Generale is surprised by the declarations apparently made by a police officer to a judge in charge of a case brought by Jerome Kerviel given that he (Kerviel) himself told detectives questioning him in 2008 that he had acted alone and without his superiors' knowledge," added the statement.

The bank also stressed that it did not have access to the legal documents from which these declarations were taken.

Kerviel was convicted in 2010 of breach of trust, forgery and entering false data in relation to the unauthorised trades, and was sentenced to three years in prison and a fine of 4.9 billion euros -- the same amount that he lost at the bank.

He was imprisoned after his conviction and sentence were upheld, although an appeals court overturned the order to pay back the bank.

He left prison in September after winning conditional release that forced him to wear an electronic bracelet at all times and stay at home every workday evening.

Contacted by AFP, Kerviel's lawyer David Koubbi was not immediately available for comment.