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Sobig-F has proved the most prevalent malevolence according to both companies. Even though it was only discovered around the 21 August, it holds the top spot in both charts and accounted for more than 60 per cent of reports, according to Kaspersky.

'August 2003 will be remembered as one of the worst months in the history of computer security. A series of different viruses have bombarded computer systems around the globe,

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culminating in the newest member of the Sobig family which swamped Internet email traffic and took the pole position in the chart. The top four viruses are all new entries - any of which would have been number one in a normal month,' said Graham Cluley, senior technology consultant at Sophos Anti-Virus.

In spots two to four in Sophos's chart reside Blaster-A, Nachi-A and Mimail-A respectively. Nachi-A is in fact a benevolent worm, which cleans up after the Blaster virus, protecting infected systems from being commandeered to launch a denial of service attack.

Beyond the fourth position, the chart makes familiar reading, with Yaha, Klez and BugBear variants all putting in appearances.

Sophos also reported the top ten hoaxes, led by the Hotmail chain letter, which urges recipients to forward it to other Hotmail users to prevent account deletion. Number two is JDBGMGR, which urges recipients to delete parts of their system. Hoaxes relating to Bill Gates also appear twice in the top ten.