Sober jumped straight to the top of the virus charts and accounted for nearly half of all viruses detected in November.

The Sober Z virus accounted for over 42 per cent of all detections last month according to anti-virus specialist Sophos. It has knocked Netsky P from the number one position that virus has held for the last four months.

"Since we saw the first Sober worm back in October 2003, its author has tried to improve upon tried and tested tricks to dupe computer users into launching infected attachments," said Carole Theriault, senior security consultant at Sophos.

"This latest worm purports to be a warning from CIA and FBI agents, accusing recipients of visiting illegal websites. Mocking the feds is a sure-fire way of goading the authorities, and you can't help but wonder whether the author is desperate to be caught."

Sober worms are frequently bilingual, configured to spread in both English and German. As well as posing as communication from an FBI or CIA agent, Sober Z also references the German version of 'Who wants to be a millionaire' as well as US star Paris Hilton.

The figures were backed up in a similar report from security hardware vendor Fortinet. It found that Sober accounted for 23 per cent of the viruses detected in November, but it still made the top spot.

This month’s most striking trend is of course the return of Sober, which, with one variant, skyrocketed in the charts," said the company.

"Truly, no malware has produced such a staggering outbreak this year. Although the monthly top 10 ranks it first, accounting for 24 per cent of November's activity, it does not do it justice."