Website reporting service Down Detector has collated at least 18,000 complaints originating from London, Birmingham, Guildford, Coventry, Manchester, Ruislip, Sheffield, Leicester, Glasgow, and Bristol. At the time of writing, it appears that the outage has been caused by a DNS issue, which allows BT to route customers' connections to websites and online services. However, some users have reported that switching connections to Google's DNS servers fixes the issue.
We have contacted BT for clarificiation but haven't yet received a response. We will update the article once we do.
Update: BT has issued a statement confirming that it has now got most of its customers back online.
"Large numbers of customers have been experiencing temporary issues with their broadband services this afternoon. Customers can still receive and make calls as normal. We've been working hard to fix the issue and are glad to report that nearly every customer affected is now reconnected, approximately two hours after the problem started.
We apologise to any affected customers for the inconvenience. There is no evidence at this stage to suggest that we were subject to a malicious attack."