Vint Cerf, one of the 'fathers' of the Internet, says up to a quarter of online computers may be used as 'bot nets' by hackers and virus writers, who have taken control after infecting them with Trojan Horse viruses.

An estimated 100 to 150 million of the 600 million computers connected to the Internet are thought to be infected.

Technology writer John Markoff told the BBC:

"It's as bad as you can imagine; it puts the whole Internet at risk."