In an online survey commissioned by security firm Akonix, a quarter of users admitted they see IM as the perfect vehicle for office gossip.
Another quarter say they have used it to send something their boss wouldn't approve of.
And 16% have sent or received sensitive company information via IM.
IM is most popular with users aged 18-29 and 80% of people in this age group say they use it to chat to friends and family while at work.
The majority of IM services - 74% according to research firm Gartner - find their way on to office PCs as a result of employees downloading them for their own purposes.
There was a risk in having software installed on PCs that has not been vetted by the IT department, said Bill Harmer, managing director of Akonix.
IM could bring big benefits to firms, he said, but it needed policies to regulate its use and technical solutions to prevent IM being targeted by spammers and virus writers.
Sixty-two percent of firms admitted to having no policy or technology in place to manage or block IM.
"The findings of this survey should be a wake-up call to UK companies. IM should be embraced but protect your business adequately or the consequences can be severe," said Mr Harmer.
Research firm Gartner predicts that, by 2006, the number of IM messages sent will outweigh the number of e-mail messages.
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