The FBI yesterday warned of imminent worldwide internet disruptions caused by a particularly virulent computer worm known as Code Red which caused disruptions in the White House and Pentagon web sites earlier this month.
US government experts said the worm, which works its way into a target computer and then uses it as a base to mount attacks on official websites, had already infected hundreds of thousands of computers and represented a worldwide danger.
The worm is now poised to emerge from its dormant phase and begin to spread rapidly by colonising more machines and sites to extend its grip.
Ron Dick, head of the FBI's national infrastructure protection centre, appealed to computer users to take precautionary measures by downloading "patches" to protect their software.
At a press conference in Washington yesterday, he said Code Red would spread exponentially and could slow down the internet.
"There is reason for concern that mass traffic associated with the worm's propagation could degrade the overall functioning of the internet and impact ordinary users," Mr Dick said.
Over the first 20 days of each month, the worm spreads to all the vulnerable computers it can find through the web, downloading itself when unwitting users try to access any web page it has hijacked.
On the twentieth day of every month, the worm is pro grammed to mount a mass "denial of service" attack on the White House and other government websites. The first documented attack took place this month, but the White House ducked the attack by changing its numerical internet address. The Pentagon also suspended public access to its site to remove the worm.
However Code Red is expected to start spreading again at 1am BST tonight (midnight GMT), marking the beginning of August.
"What makes this one different from any other is how dramatically ... it has been able to propagate itself and the viciousness associated with that," Mr Dick said.
The worm targets Microsoft's internet information services (IIS) software versions 4.0 and 5.0 as well as NT and 2000 systems .
So far, Windows 95 and 98 platforms have been immune. Microsoft has made a software patch on its website, designed to immunise computers against attack.
The worm defaces web sites by putting the phrase "Hacked by Chinese" up on the screen. However, the origin of the worm attack remains unclear.
The initial strain of Code Red did not destroy files on the computer it infected but a joint statement issued yesterday by the FBI, Microsoft and university computer experts said the bug "has mutated so that it may be even more dangerous."
The statement warned: "This spread has the potential to disrupt business and personal use of the internet for applications such as electronic commerce, email and entertainment."
