Germany's Deutsche Bank said it took a charge of around €450m last year in relation to share trading fraud, but declined to give any details.
The bank increased its provisions for "external fraud" to €475m in 2015 from €20m in 2014, according to its annual report.
"The increase in the event type 'External Fraud' is caused by a provision for equity trading fraud," the bank said in the report, which was published in March.
A spokesman declined to say what event or events the charge related to or when it occurred.
The previously unreported charge was included in Deutsche Bank's operational risk disclosures.
Under Bank for International Settlement operational risk definitions, "external fraud" relates to "Losses due to acts of a type intended to defraud, misappropriate property or circumvent the law, by a third party".
Most other big banks do not break out the cash value of operational risk losses.
