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.