Almost half of UK businesses have been affected by computer viruses over the past two years; two-fifths of these were described as having a ‘serious impact’ on business.

A telephone survey of 1,000 companies showed virus infections were more likely to have caused service interruption than other incidents. Usually the disruption was minor, but roughly a quarter of companies questioned who reported a virus as their worst incident had major disruption, with important services such as email down for more than a day.

The Department of Trade and Industry’s survey showed the majority of UK businesses surveyed have a broadband link to the internet (88%) and as a result, the threat from as viruses has never been greater. UK businesses have responded and now almost every company uses anti-virus software. Despite the increased threat, fewer companies had viruses than in the last two surveys. Infection rates have dropped by roughly a third since two years ago.

While the number of companies infected has fallen since 2004, the average number of infections suffered by those affected has risen to roughly one a day.

Key findings from survey include:

A quarter of UK businesses are not protecting themselves against the threat caused by spyware. As a result roughly one in seven of the worst incidents involving malicious software related to spyware that can download onto a computer when the user visits an unscrupulous website.

Two years ago, a small number of viruses dominated, for example Netsky and Bagle/Beagle. In contrast over the last year, no single virus has caused widespread damage. Instead the nature of viruses - and the motivation of their writers - has changed. Some malicious viruses, known as 'bots', take over machines turning them into 'botnets" used for cyber crime and cleaning up the problems can take weeks of effort.

Patching discipline has improved: nearly nine in 10 UK businesses (88%) apply new operating system security updates within a week of their release, compared with 79% of businesses in 2004.

Companies without anti-virus software did not report many infections. One explanation is that companies that suffer virus infection tend to install anti-virus software afterwards.

Virus infections tended to take more effort to resolve than other incidents sometimes taking over 50 days' work to fix.