A hybrid new computer worm was last night posing a serious threat to millions of commercial servers and home PCs.

Reports of the worm, known as Nimda, were being received from all over the world by computer security experts in Britain who recorded more than 45,000 cases in five hours.

The worm contaminates emails and infects websites so that when a user visits a compromised site, the browser can infect the PC. Nimda is thought to be the fastest spreading virus to date. It arrives in email without a subject line and containing an attachment: "readme.exe".

Jason Holloway, UK general manager of security software company F-Secure, believes the worm could be more destructive than the Code Red worm that affected 250,000 machines in days. "That was an internet worm and this one is a hybrid. It is ingenious from a technical point of view but it is very, very worrying," he said.

US Attorney General John Ashcroft said last night there was no evidence to link Nimda with terrorist attacks.