BT customers suffer email failure in account migration
Tens of thousands of BT customers were left without email over the weekend, the company has said.
Users complained they could neither send nor receive messages from Friday 11 July, with "thousands" still affected on Monday afternoon.
BT said there was a problem with the transfer of customers from Yahoo to its own mail service.
Some users complained that there had been insufficient communication from the company.
BT also admitted that some emails had been lost as a result of the problem.
In a statement, the company apologised and said it was told of the problem by Openwave Messaging, which is handling the migration of around seven million BT customers, on Saturday night.
That meant that a "minority of BT's customers would have seen an error when trying to log in and were unable to access their email", the statement read.
A spokesman told the BBC that the total number of people affected was "in the tens of thousands". While the majority of those have had their service restored, the number still without remained in the thousands, the spokesman said on Monday.
Some customers complained that they had been given little information by BT as the problem persisted over the weekend.
"For the size of the service provider, one would have expected some sort of statement on their website," David Hunt, from Nottingham, told BBC News.
He said that he has filed a complaint with BT. He said that people who needed to access their emails over the weekend, such as those who were using e-tickets they had not downloaded, would be stuck. "I can see how that causes problems," he said.
Other customers wrote to the BBC to complain. Alan Ratcliffe, from Weybridge, Surrey, said he was forced to use a rival service and suggested that, if the problem persisted, others might do the same more permanently.
It is the second time BT has been forced to apologise in a matter of weeks. Late last month, it did so after admitting that "many" of its UK users experienced problems connecting to the internet.
In March, it emerged that BT was being investigated by the Information Commissioner amid claims that the company "exposed user credentials en masse".
A whistleblower came forward with concerns about BT's security as it handled the migration of its customers' email accounts.