Security firm Sophos said that more than a fifth of the world's spam originated from the US during the last quarter.

The company believes that a possible reason for the country’s increasing lead in relayed spam is the emergence of more than 300 strains of the mass-spammed Stratio worm. The worm, also known as Stration or Warezov, uses a trick dependent on the victim being able to speak English in its attempt to convert innocent PCs into members of a spam botnet.

After the US, which accounts for 21.6% of relayed spam in Q3 2006 in Sophos' list comes China (13.4%), France (6.3%), South Korea (6.3%) and Spain (5.8%). The UK has successfully dropped out of the top 10 chart, and is currently in 13th position.

Most unsolicited emails are now sent from zombie PCs – computers infected with Trojans, worms and viruses that turn them into spam-spewing bots – says Brett Myroff, chief executive of Sophos distributor, NetXactics.

"In the past, hackers relied on operating system vulnerabilities to convert innocent computers into zombies. Now, they are turning back to malware to trick users into running their malicious code, and opening the backdoor to hackers." Hundreds of new versions of the Stratio worm have helped steadily increase the volume of spam seen travelling across the net.

"Integrated anti-malware and anti-spam protection is getting the better of illegal spam peddlers – forcing them to get more creative and crooked. However, if people are playing their security cards right, the spammers' efforts will still be in vain," says Myroff.