SoBig.F has shot to the top of the virus charts in what the security industry has described as one of the worst months on record.

Many antivirus 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, commented: "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, and their impact on organisations and productivity."

Graham Cluley, senior analyst at antivirus vendor Sophos, added: "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 that SoBig.F 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 to 60 per cent of infections in most antivirus firms' charts. Blaster.A, Nachi.A and MiMail.A averaged from five to 10 per cent of infections, and Klez.H is still in the top 10.

In one chart Klez.H has been in the top 10 for 19 months.

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