Businesses yesterday rushed to secure computer systems as a rash of competing viruses targeted corporate networks around the world running some versions of Microsoft's software.

Companies affected included CNN, ABC News, the New York Times and the Financial Times. Computers at DaimlerChrysler, Kraft and UPS were also reported to have been infected by the worms, known by several names including Zotob and Rbot.

"We have grouped them into four main families," said Mikko Hypponen, chief research officer at Finnish security software company Datafellows. "They're even fighting with each other; some versions are removing earlier versions. It seems like some fighting is going on within rival virus-writing gangs."

By last night 12 variants of the worm had been identified and Mr Hypponen said more strains would probably be released over the next few days. However, the threat was expected to recede as companies updated software.

The impact is thought, so far, to be less than from similar attacks by Blaster in 2003 and Sasser last year.

"As people patch their systems and start to take precautions, [ the threat] will diminish," said Les Fraser, security spokesman for the British Computing Society.

However, Ken Allan, partner at Ernst & Young, said the cost could run into millions of pounds.

The rash of worms was prompted by Microsoft's publication last week of details of a vulnerability in Windows 2000, Windows Server 2003 and some versions of Windows XP.

The day after the vulnerability was publicised an anonymous programmer, believed to be in Russia, published a computer code on the internet that made it easy to exploit the flaw.