The information, which included names, addresses, telephone numbers, bank and credit card account numbers and even signatures, was stored on the hard drive of a laptop bought by IT manager Andrew Chapman. Chapman purchased the machine from Graphic Data, a company specialising in creating and holding digital copies of paperwork for financial institutions, for £35.
It is believed an ex-employee is responsible for selling the laptop. RBS has also admitted that another laptop has gone missing from its offices in Shoeburyness, Essex
"Any breach of these procedures is totally unacceptable and is investigated as a matter of urgency," said a spokesperson for RBS.
The Information Commissioner's Office is investigating the incident, which comes just a month after the MoD admitted that 658 laptops have been stolen or lost since 2004.
See also: HSBC loses 370,000 customers' details