Many anti-virus manufacturers detail the number of infections recorded on a monthly basis and have reported four major new worms.

Despite the harm caused by the Blaster virus, it was left trailing by SoBig.F, which at its worst was responsible for nearly three-quarters of all email traffic.

Steven Sundermeier, vice president of products and services at Central Command, said in a statement: 'The month of August turned into a plague of internet worms affecting computer users worldwide.

'Multiple aggressive internet worms, made it the worst month in history for the number of infections reported, impacted organisations and lost productivity.'

Graham Cluley, senior analyst at antivirus vendor Sophos, added in a statement: 'August 2003 will be remembered as one of the worst months in the history of computer security. The top four viruses in our chart are all new entries - any of which would have been number one in a normal month.'

Sundermeier claimed SoBig had left networks in danger of collapse. 'The extremely aggressive spreading nature of Sobig.F created significant volumes of email traffic causing email networks around the world to collapse,' he said.

SoBig.F accounted for 30-60% of infection in most AV firms charts. Blaster.A, Nachi.A and MiMail.A averaged about 5-10% of infections and Klez.H is still in the top ten and has now been on one chart for 19 months.

Central Command updated its Vexira Antivirus product for 816 different viruses, worms and other malicious applications during August.