Cybertrust's ICSA Labs has completed a survey suggesting that the number of virus attacks last year grew by more than 50 percent. The figure was based on the number of attacks and costs to businesses affected by them.

CSA said that the numbers had increased again for the tenth consecutive year making a figure of 392 "virus encounters" per 1000 computers per month last year. The number of infections per 1000 PCs increased to 116 per month.

The number of "virus disasters," where 25 or more PCs or servers were infected at the same time in an organisation, was up 12 per cent from 2003.