The Bank of Ireland has lost the personal details of nearly 900 customers.
The sensitive data, which includes telephone numbers, addresses and accounts details, had been copied onto a portable flash drive, which was subsequently lost by an employee.
According to security vendor Sophos, the theft, which was discovered last week, provides cyber criminals with enough information to commit ID theft.
"Bank of Ireland has informed those affected about this latest security blunder, and has promised to monitor their accounts for unusual activity," said Graham Cluley, senior technology consultant at Sophos.
"This security lapse should never have happened in the first place," added Cluley.
The Data Protection Commissioner is currently investigating the incident.
In April, the Bank of Ireland reported four laptops also containing personal information of over 31,000 customers had been stolen. The laptops have yet to be recovered