The company has had to report the hack to the UK’s data watchdog, the Information Commissioner’s Office, after hundreds of its customers vented their anger in online forums.
Yahoo! provides email services to BT’s 6m UK broadband customers.
One entry in the BT customer forum reads: “I ALWAYS use unique passwords for everything, ALWAYS have at least 8 characters with a mixture of numbers. No-one has access to my computer or can see my passwords anywhere. My BT password was very strong. Yet I got hacked! My contacts received bogus emails containing links and other rubbish. Shame on you BT. Your security is an utter disgrace.”
BT confirmed it was aware of the incident and said that customers who have been affected will be warned when they try to log in to their account.
The Information Commissioner’s Office was not immediately available for comment.