Virgin Media has lost the bank details of 3,000 of its customers.
According to the ISP, the details, which were on an unencrypted disc, would usually be sent by FTP. It is now investigating the problem in a bid to discover why the files ended up on an unencrypted disc.
Virgin Media said it had notified the Information Commissioner's Office, as well as ensuring relevant fraud bodies were notified so that customer bank accounts would be closely monitored for illegal activity.
"At this stage, we have no evidence the disc has fallen into the wrong hands," a spokesperson said.
In a statment, the company said: "This is an isolated incident which has affected a small number of our customers. We have been working with the Information Commissioners' Office on this matter and we are in the process of contacting all of the affected customers to ensure we meet our responsibilities and fully support them through this process.
"Customer privacy is of the highest important to us and we are undertaking a full review of our data protection policies and practices to ensure this matter does not occur again. We are very sorry this situation has occurred and for the inconvenience this has caused our customers."
Theft of unencrypted information has been in the headlines recently, after the NHS lost the details of 31,000 patients on seven unencrypted laptops. Meanwhile, Cotton Traders was reported to have lost the details of up to 38,000 customers following a hack of its website, but would not confirm whether the data was encrypted or not.