British multinational telecommunications services company BT and Internet firm Yahoo are probing surge in reports of email accounts being hijacked by criminals for sending spam.
Yahoo provides email service to BT's over six million broadband customers.
The customers had complained their contacts have been spammed and their passwords changed so they are locked out of their account.
Login records showed that BTYahoo accounts have been accessed via computers in Turkey and Russia, the Telegraph reports.
BT denied speculation by some customers that the increase in hijacking over the last week was the result of a security breach on its network.
A spokeswoman of the firm said that BT is 'committed to protecting their BT Yahoo and BT Internet email accounts and it ants to assure customers that there are no security issues in terms of BT's network or systems.
According to the paper, Yahoo said it was "aggressively" investigating the problems.