A new Internet computer worm has infected millions of computers causing major disruptions over the weekend.
Computer security experts have warned that the virus may spread rapidly when businesses resume work on Monday morning.
Named Sasser, the worm, began to spread on Saturday, and unlike a virus does not travel through e-mails or attachments.
It can spread by itself to any unprotected computer linked to the Internet.
The virus attacks through a flaw in recent versions of Microsoft's Windows — Windows 2000, Windows Server 2003 and Windows XP — and causes the computer to shut down, then rebooting it, repeating the process several times.
However experts say it appears to do no lasting damage.
One anti-virus company said that slightly more than three percent of the world's computers, around 18 million out of the estimated 600 million operating worldwide, were infected.
"Compared to other uiruses which have appeared on weekends when activity is low, this one has positioned itself as one of the quickest-spreading and most virulent ones," Luis Corrons of PandaLabs, which has offices in Spain and the United States.
"The problem seems to be getting worse," Mikko Hyppoenen, an anti-virus expert at F-Secure, a leading internet security firm, told AFP from Helsinki, adding that millions of computers worldwide may have been infected.
Laptops are most vulnerable as they are not protected by company firewall systems.
If used on a server other than the company's, they run the risk of being infected and in turn infect the company's network when used in the office.
"It seems to me an exaggeration to say that millions of computers have been affected," Bernard Ourghanlian, Microsoft's technical director in France, told AFP, where work was disrupted by the worm on Saturday night.
But he acknowledged that the worm was spreading Sunday.
Microsoft made a software update available last month to fix the flaw exploited by the worm, and since mid-April several million copies have been downloaded.
