Virgin Media says it is investigating a problem which has left many customers in London without internet access and TV services.
The company tweeted that the problem “might be fixed this evening”, but said a separate “system issue” left it unable to provide more information.
The problems were first reported on tracking website Down Detector at about 16:00 BST.
Virgin Media said engineers were working to fix the “power issue”.
It said the problem was confined to London and was not a nationwide issue.
However, several customers outside the capital including Maidenhead and Birmingham complained about problems on social media.
Virgin Media’s customer support line is currently so busy that the company has advised people to call another day.
The company said on Twitter that customers could apply to receive “credit for any loss of service”.
A Virgin Media spokeswoman said: “We are aware of a power issue impacting our services for customers in some parts of London – this is not a nationwide issue.
“Engineers are on-site and are working to restore services as quickly as possible. We apologise for any inconvenience caused.”