A number of leading websites are currently not working, including Amazon, Reddit and Twitch.
The UK government website - gov.uk - is also down as are the Financial Times, the Guardian and the New York Times.
Affected websites displayed the message: "Error 503 Service Unavailable".
Early reports suggested it could be related to Fastly, a cloud computing provider, which underpins a lot of major websites.
Fastly said it was looking into problems with its global content delivery network (CDN).
In an error message posted just before 11am BST Fastly said it was "currently investigating potential impact to performance with our CDN services."
It has not yet provided a formal statement.
