The data was stolen from Cable & Wireless in December 2005 by a third party which the company believes it can identify. Bulldog's customer base has since been sold to broadband provider Pipex, but C&W is investigating the breach.
James Brown, managing director of Bulldog Internet, told the Guardian newspaper: "Our understanding is that, following an external enquiry by Cable & Wireless, it has become apparent that at some point in December 2005 Cable & Wireless had some of their customer contact details illegally obtained by a third party. This resulted in a small number of their customers receiving unsolicited calls."
C&W said that it was preparing legal action against a third party which it said could be the source of the leak.
It is not yet clear exactly what customer data was taken. Several customers have reported receiving telephone calls that alerted them to the security breach. It is not known whether or not credit card or bank details were among those taken. C&W said that there was no evidence that that was the case.
Large scale data thefts are becoming increasingly common as identity theft becomes a more lucrative crime. With individuals carrying out more and more of their economic activity online, impersonating those people can bring ever greater rewards.
The US has been the location of the most serious data breaches. One recent US breach had implications for UK citizens, though. The owners of High Street discount clothes chain TK Maxx suffered one of the biggest ever breaches when the credit card details of 45 million customers were stolen by a hacker.
In a regulatory filing last month the shop's parent company, TJX Companies, said that data had been stolen in the UK. "We believe that information was stolen in the computer intrusion from … a portion of our computer systems in Watford, U.K. that processes and stores information related to payment card transactions at T.K. Maxx in the United Kingdom and Ireland," said the filing.
