A French judge has filed preliminary charges against a trader suspected of losing more than €600m ($751.5m) in complex derivative trades at French mutual bank Caisse d'Epargne.

The trader, Boris Picano-Nacci, was placed under judicial supervision by investigating magistrate Xaviere Simeoni after spending 36 hours being questioned by financial police, the judicial official said on Thursday.

The move bans Picano-Nacci from leaving the country or having any contact with employees of the bank and gives the judge time to investigate before ordering a trial or dropping "breach of trust" charges.

The bank's top three executives quit after the losses became known on October 17.

A Caisse d'Epargne spokesman said the bank now calculates the loss was larger than the €600m it initially announced, and although he declined to provide a new estimate, the prosecutor's office said the loss would amount to €751m.

Christine Lagarde, the French finance minister, said last week that an initial investigation discovered "serious holes" in the bank's system of controls.

The bank's own internal investigation found a large number of breakdowns in internal controls and said alerts had been disregarded, French news magazine Nouvel Observateur reported on its website on Wednesday, citing a copy of the investigator's report.

The losses drew comparisons with the much larger trading scandal suffered by another French bank, Societe Generale, earlier this year.

Societe Generale took a €4.9bn hit closing what it says were unauthorised positions by former trader Jerome Kerviel.