Large firms are each losing up to £2.5 million a year due to workers using office computers to surf the net for their own purposes.
According to a recent report, nine out of ten workers have unlimited access to the internet and e-mail systems, which are considered vital business tools.
New Government regulations giving bosses power to monitor employees' e-mails and phone calls came into force last week.
Clare Hogg, author of the report, said the scale of the problem was "huge".
She warned employers should have solid policies in place so workers were aware of the consequences of misuse.
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