Around 100,000 internet users have had their personal details stolen from broadband provider Bulldog.
The theft took place in December 2005 when the company was owned by Cable & Wireless.
Cable and Wireless is investigating the security lapse although Bulldog’s customer base has since been sold to Pipex.
It is not clear what customer details were taken but Cable and Wireless has said there was no evidence that credit and debit card details were stolen.
The company has said it is ‘taking appropriate legal action’ against the third parties that it believes may be responsible for the ‘unauthorised use of our customer data.’
The security breach emerged just weeks after it was revealed that crooks had stolen 45.7 million credit and debit card numbers from the American retailer that owns bargain chain TK Maxx.
The data was accessed on TJX’s systems in Watford and Massachusetts over a 16-month period and covered transactions dating as far back as December 2002.