Around one in five companies in the UK has experienced problems with employees misusing the internet, according to official figures.
Over the last year nearly two-thirds of large companies were affected, reporting an average of one incident a week.
The results, revealed in the 2004 Department of Trade and Industry biennial Information Security Breaches Survey, found that the two biggest areas of misuse were excessive personal emails and accessing inappropriate websites.
Researchers conducted 1,000 telephone interviews with organisations of all sizes and a number of face-to-face interviews between October 2003 and January this year.
They found that employees had web access in almost nine out of ten of all UK businesses and almost all (98 per cent) larger companies. When the survey was last carried out in 2002 the figure was only seven out of ten.
Considerable resources are also expended on investigating Internet misuse; investigations into incidents of misuse typically take up to a week to complete, the research found.
Chris Potter of PricewaterhouseCoopers, which carried out the research, said: "As more businesses provide their staff with access to the internet, the number of incidents of staff abusing that access is rising. It seems unwise to wait until a major breach before putting effective controls and plans in place."