Twitter has been down for several hours following a denial-of-service attack on the site. The company announced the problem on its status blog, initially saying that it was trying to determine the cause, before confirming the DDOS attack.

At the time of writing, the site is back up for some but running very slowly. Antivirus company Sophos explains what a DDOS is: "A denial of service attack occurs when computers bombard a website with requests for information".

"Typically hackers can control thousands of innocent users' computers centrally and command them to visit a site that they wish to flood with traffic - making it impossible for other internet users to get through. It's a bit like 15 fat men trying to get through a revolving door at the same time - nothing can move".

The site is expected to be up and running again for everyone again shortly.

Update: The site appears to be back up, complete with a tweet from site founder Biz Stone (@biz) saying: We had a lot of things we'd rather be doing this morning, defending against a DoS wasn't one of them".