A power failure is causing problems for BT broadband customers across the UK, the firm said.
The fault at the major exchange in Birmingham has meant a loss of internet service for BT customers in several parts of the country, including Cardiff, Bristol, East Yorkshire and London.
BT said the "majority" of customers' broadband services had already been restored and for most the problem would have lasted for "little more than an hour".
"We can confirm that, as a result of a power failure at one of our major exchanges, some customers may currently be experiencing loss of broadband service," a spokeswoman said on Monday.
"Our engineers are on site and the majority of customers' service has already been restored.
"We are working to restore service to remaining customers as soon as possible this afternoon.
"Should any customers continue to experience difficulty in accessing their broadband service, they are advised to turn their hub or modem off and on again."
BT said the "brief outage" at the exchange in Birmingham affected hundreds of thousands of customers.
However, the company stressed this was no more than 5% of the total broadband connections over its network.
"We can confirm that, as a result of a power failure at one of our major exchanges, some customers may have experienced loss of broadband service for a brief period this afternoon," a spokeswoman said. "All services have now been restored, with the majority of BT's consumer broadband customers' service being restored within just one hour.
