BT has urged people who use BT Yahoo Mail to change their passwords after Yahoo revealed it fell victim to a second hacking attack in 2013.
Yahoo had already confirmed earlier this year that it experienced a data breach back in 2014.
However, it has now disclosed that another breach had occurred a year earlier.
BT has outsourced its webmail to the Canadian technology firm, which means millions of its customers could potentially have had their details compromised.
As a result, it has recommended that anyone who uses BT Yahoo Mail and has not changed their password since August 2013 do so straight away.
Confirmation of a second security breach at Yahoo has prompted widespread criticism, with US Democratic Senator confirming he now plans to investigate the company's cyber security arrangements.
"This most-recent revelation warrants a separate follow-up and I plan to press the company on why its cyber defences have been so weak as to have compromised over a billion users," he said.