The information was available from a simple Internet search and included the names, home addresses and full card details of thousands of Visa, Mastercard and American Express customers, the Daily Mail reported Saturday.
The newspaper said the Internet search engine Google picked up the information, allowing it to be seen and copied.
The banking industry trade body APACS said many of the credit cards on the list had already been stopped, while others had expired. Many of the affected customers have not been warned by the financial institutions, the newspaper noted.
"This is hugely worrying. The credit card companies have a duty of care to inform all those involved that they are at risk of identity fraud," said Negel Evans, a member of Parliament and chairman of the All Party Group on Identity Fraud.
The newspaper said the credit card information was stored on an unsecured Internet server in Vietnam.
"To find this amount of data on a server which is publicly accessible is a rare event. Organized crime usually protect their ill-gotten gains behind password-protected links on encrypted machines," said Rik Ferguson, of web security firm Trend Micro.