The Sober-N worm generated nearly half of all virus strikes in May, according to figures released today by Sophos.

The worm went straight to number one in the company's monthly top 10 listings, with the second-placed Zafi-D reaching just 14 per cent of recorded attacks. The only other new entry for the month was Mytob-AZ at 1.6 per cent.

"Sober-N stormed to the top of the chart in early May, making it one of the biggest outbreaks so far this year," said Carole Theriault, security consultant at Sophos.

"This manipulative email worm spread quickly using social engineering tricks, such as offering free World Cup tickets, to entice recipients into opening the infected attachment.

"The Sober-Q Trojan, released a few weeks later, searched for computers infected with Sober-N and attempted to turn them into spamming machines."

Sober-N helped to increase the amount of virus activity last month to 2.6 per cent of all emails sent, up from 2.2 per cent the previous month.