Businesses in Asia were the first to report the attack. By mid-day in Europe, tens of thousands of computer users had been hit, with more cases expected in North America as the business day wears on.

UK-based virus detection firm MessageLabs recorded 17,765 cases in 24 hours. "We've upgraded it to high-risk just for the fact that we've seen so many in the last day," said Mark Toshack, a virus analyst at MessageLabs.

The worm, which carries random subject lines, disables computer users' anti-virus and firewall software, but is otherwise not a threat to users' personal files. The biggest headache was the extra traffic it generated, bogging down corporate networks.

The mass-mailing worm spreads via email, online chat sessions and in peer-to-peer file sharing network such as Kazaa. It affects computers using the Windows operating system.

"It sends an email message with varying format to all the addresses found in the Windows Address Book and Microsoft Outlook," Japanese security firm Trend Micro said.

The worm arrives as a file attachment with a .EXE, .PIF, .COM, or .SCR extension.

Other security software makers issued similar warnings through their Web sites, including U.S. firm Symantec Corp (www.symantec.com) and Finland's F-Secure (www.f-secure.com/v-descs/fizzer.shtml).