Halifax Bank of Scotland (HBOS) is investigating how an Aberdeen woman who asked for a bank statement was sent account details of 75,000 other customers.
Stephanie McLaughlan, 22, received five packages, each containing 500 sheets of 30 customers' names, sort codes and account details.
HBOS has apologised for the error and has said it is carrying out an investigation into the "serious" but "isolated" incident.
McLaughlan told the BBC website that she would now probably change to another bank, because of HBOS’s “terrible negligence”.
HBOS was formed as a result of the merger between the Halifax and Bank of Scotland.
It is not clear whether the bank will be contacting each customer whose details were listed in the five statement packages wrongly sent out.
In the US, where mislaid or lost customer details from banks and retailers are now being constantly reported, those affected are usually individually contacted and offered free credit checks.
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