The number of bank staff stealing or deceiving customers has risen by more than 40% in the last year, fraud figures show.
CIFAS, an anti-fraud organisation whose members include most major banks and building societies, said that there had been an "alarming increase" in the level of fraud committed by employees.
One particular trend was for cashiers to cream off elderly customers' deposits when they take cash into a branch.
According to CIFAS, the total number of insider fraud cases identified rose from 330 in 2010 to 378 in 2011, an increase of almost 15%.
However, the number of dishonest actions by staff to obtain a benefit through theft or deception rose from 156 cases to 220 - a 41% increase.
In order for a fraud to be entered onto the CIFAS database, the CIFAS member must investigate and reach a 'burden of proof' before filing.
CIFAS's Richard Hurley said: "Many of these fraudsters steal from elderly and more vulnerable account holders. The perpetrators' actions are as bad as muggers in the street."
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