Viruses like Klez and Bugbear are still in circulation even though fixes have been available for months - and the UK's small businesses may be to blame.

A survey of UK small and medium-sized enterprises by antivirus vendor Sophos found that fewer than half (46 per cent) of the businesses questioned had any virus protection at the gateway to their networks, and that only 42 per cent updated their antivirus software more than once a week.

To date, Klez is still the most common virus detected, although protection against it has been available for nearly a year. Bugbear is the second most common and a fix has been available for the last four months.

But the viruses keep recurring because unprotected systems act as incubators and periodically release new infections.

With an average of 150 new viruses, typically variants of existing threats modified to slip past filtering software, detected each week, the need for regular updates on antivirus software is crucial.

"Updating your antivirus software only once a week is like brushing your teeth only once a week: it only gives you the minimum protection and could lead to painful consequences in the future," warned Graham Cluley, senior technology consultant at Sophos Anti-Virus.