Security experts say that the MyDoom virus that spread rapidly across the internet last week is one of the costliest cyber attacks yet.

The virus reached more than 160 countries and peaked at one in ten of all emails sent worldwide, according to analyst Gartner.

Security firm mi2g estimates that the cost of the virus is more than $38bn, based on the factors such as of loss of business, bandwidth clogging, loss of productivity, cost of recovery and software upgrades.

The virus was designed to disable the Microsoft and SCO Group web sites, which led both suppliers to offer a $250,000 reward for catching the hacker responsible. The offer did not prevent SCO's web site from being taken offline on Sunday. As Computing went to press, MyDoom-infected PCs were expected to attack microsoft.com, although early indications suggested the effect would be less dramatic than the SCO attack.

'The MyDoom outbreak is one of the fastest-spreading attacks in history and may prove to be one of the most dangerous as well,' said Gartner analyst John Pescatore.

'This sophisticated, well-engineered worm comes in a small package that can infect many machines before being detected. Gartner believes that this type of "blended" attack is now the most serious malicious-code threat facing enterprises.'