About 50% of UK businesses were attacked in 2003, up from 41% in 2002 and 16% in 2000, the survey of 1,000 companies due to be published next month found.
Despite the widespread take-up of anti-virus software, viruses were the most damaging security breach in 2003 for 66% of companies. More than 70% of businesses and 83% of large companies received infected e-mails in 2003, with large firms experiencing an average of 12 virus attacks a year. In some cases the attacks brought systems down for more than a month.
The findings suggest that many firms rely heavily on anti-virus software but fail to protect their systems from worms through regular patching.
The emergence of worms such as MS Blaster, which was responsible for 56% of the virus infections in large companies last year, has put pressure on businesses to speed up their patching process.