Computer viruses are the single biggest cause of security problems for UK businesses, a survey by the Department of Trade and Industry shows.
The study found almost 50% of the biggest security breaches suffered by companies in the last two years were due to infection by malicious programs.
In some cases viruses crippled key systems such as e-mail for more than a day while companies cleaned up.
It also found that the worst outbreaks can take up to 50 days to fix.