Half of UK businesses were hit by viruses in the past year, according to the initial findings of the Department of Trade and Industry's biennial Information Security survey.

This figure rises to 68 per cent among larger companies, which suffered most damage from the Blaster worm designed to launch a denial of service attack against Microsoft.

Chris Potter, a partner at PricewaterhouseCoopers, which is conducting the survey for the DTI, explained that Blaster had proved costly and difficult to eradicate in large networks.

The worm was less effective within smaller firms, but attachment-based viruses like Bugbear had hit harder in this group. One reason for this was the lower level of staff training in safe computing.

One in seven firms have failed to take cyber threats seriously, according to the survey.

Seven per cent of respondents still have no antivirus software in place, while a further seven per cent only update their antivirus software annually or not at all.

"While people do have antivirus software they don't always keep it up to date," said Potter.

"On the plus side 59 per cent now update automatically and that's a significant advance on two years ago."

The lack of updates and the increasing sophistication of virus writers were the main reasons for the increased number of infections.

"The newest viruses are like Swiss Army knives: if they can't get in one way they will try another," warned Potter.