Computer security experts have given warning today of a new computer virus that is expected to attack computers which are still infected with the MyDoom virus.

The new worm, known as DoomJuice, was detected last night and has so far infected at least 30,000 computers worldwide since it was activated on Sunday.

Mikko Hypponen, director of antivirus research at F-Secure, a Finnish computer security firm, said that DoomJuice, like MyDoom, is designed to attack Microsoft's main website.

Mr Hypponen said: "Unlike Mydoom, it does not spread via e-mail. It comes through a backdoor left open by Mydoom. People won't even realise their computers are being attacked, and then they'll have both Mydoom and Doomjuice in their computers."

F-Secure was one of the first to give warning of the dangers of the e-mail MyDoom worm, also known as Novarg.

Mr Hyponnen believes that the authors of DoomJuice are the same people that wrote MyDoom, which infected up to one million computers worldwide in just a few days, clogged the internet and caused huge delays in the delivery of e-mails worldwide.

The main Microsoft website appears to be operational so far, but F-Secure said that it had noticed a disruption in service yesterday.

Microsoft announced last month that it was offering a reward of $250,000 (£134,000) to anyone that helps find and prosecute the author of the MyDoom.

Doomjuice's ability to spread is limited because it will only attack computers that are still infected by MyDoom, Mr Hypponen said. "And lots of them are being cleaned up already at a quick rate."

But he warned that, unlike MyDoom which is programmed to stop spreading on February 12, Doomjuice could run forever. "At least until all computers everywhere infected by both worms have been cleaned up, and that could be years," he said.