BT has admitted that tens of thousands of its customers could not access their email accounts over the weekend,
Users could not send or receive messages from Friday 11 July, with thousands of customers still affected by Monday morning.
The company has stated that there was an issue transferring users from Yahoo to its own mail service and that some emails had been lost as a result of the problem.
BT has said that it was made aware of the issue by Openwave Messaging, the company handling the transfer of approximately seven million customers, on Saturday night. However, many users have complained about the lack of communication from BT.
In an interview with BBC News, David Hunt of Nottingham said, "For the size of the service provider, one would have expected some sort of statement on their website." Mr Hunt has filed a complaint to the corporation and many others have complained that their plans were severely disrupted by the issue.
The email breakdown is the latest in a series of problems to plague the communications giant over the last few weeks. Last month, the firm admitted that many of its UK users had difficulty connecting to the Internet.
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In March concerns were also revealed over BT's security, after a whistleblower came forward with information regarding the migration of customer's email accounts. This has led to an investigation by the Information Commissioner over claims that the firm "exposed user credential en masse."