WASHINGTON, Aug 22: A new mass computer worm that attempts to download files from the Internet and potentially leave computers vulnerable to further attack is being described by experts as the fastest e-mail virus outbreak ever.

In separate statements, at least two anti-virus companies - F-Secure and MessageLabs - said the new worm, dubbed Sobig.F, was spreading like wildfire, disrupting computers at homes, businesses and educational institutions.

The Sobig.F is at least the fourth new, major Internet worm to hit computers worldwide in the past week, prompting anti-virus vendor F-Secure to declare this the “worst virus week ever”.

MessageLabs, which scans e-mail for viruses, said that within 24 hours it had scanned more than one million copies of the latest variant of the virus.

“It’s unprecedented in our history. ... It’s a pretty frightening statistic. And the next incarnation could be even worse,” MessageLabs chief information analyst Paul Wood said while talking to reporters.

Sobig.F is the fifth version of the virus, which has had an expiration date with each variant. The prior version expired last month. With Sobig.F set to expire on Sept 10, the next version, - Sobig.G - could appear as soon as Sept 11, anti-virus vendors warned.

Sobig does not physically damage computers, files or critical data, but it ties up computer and networking resources. One in 17 e-mails sent around the world since Monday had been affected by Sobig, analysts said.

Some experts fear the virus could increase global e-mail traffic by as much as 60 per cent, slowing the Internet to a crawl.

They think that the author of the new bug may be using the worm to construct an elaborate network of hijacked computers that can be used to send spam.

The virus spreads when unsuspecting computer users open file attachments in e-mails that contain such familiar headings as ““Thank you”, “Re: Details” or “Re: approved”.

Once the file is opened, Sobig scours the computer for e-mail addresses, checking in Word documents, Internet logs and e-mail inboxes. Designed like mass-mailing spam programmes, it then sends scores of messages to the addresses it has collected.

Before Sobig.F, the previous record for an e-mail infection was “Klez”, with about 250,000 copies spotted during its first 24 hours earlier this year. There have been faster outbreaks on the Internet, but those circulated through networking functions built into Windows operating systems.

In Bangalore, dozens of cybercafes shut down and home computers blacked out. Some cafes were hit because their service provider was affected, but others got the virus in machines using Windows operating systems.

FALLOUT CONTINUES: The fallout from the “MSBlast” or “Blaster” worm continues. The outbreak began 10 days ago with the so-called “Blaster” or “LovSan” worm, which, by some estimates, infected more than 500,000 computers running the latest version of Microsoft Windows.