Network management software company Packeteer discovered the extent of the problem while demonstrating its products at 500 UK companies over the past six months.
Besides being a drain on storage assets, industry watchers have pointed out that UK businesses could be open to prosecution if they hold pirated material or allow it to be copied to CD on their premises.
The problem has already achieved a high profile in the wake of the £1m lawsuit brought by UK music industry body the British Phonographic Industry (BPI) against EasyInternetCafe.
File swapping software - typically only a few megabytes in size - is easily downloaded and allows end-users to view and obtain files such as music tracks from others. The file swapping revolution began with the now-defunct Napster service and carries on today through freeware such as Kazaa and Morpheus.
At present the BPI will not prosecute businesses whose employees contravene copyright.
A spokesman said, "Downloading copied music is illegal. Our anti-piracy unit writes to the company and informs it that abuse is taking place, following which, it is up to the company to take action against employees misusing company computers."
Analyst Eric Woods of Ovum said, "To discover such misuse is a monitoring exercise, but a cultural change is needed too because computer access has become so widespread that maintaining boundaries has become harder.
"The main ways of discovering such misuse are through spotting unusual bandwidth usage and monitoring for file swapping applications and types of files, such as MP3, which have been downloaded."
Napster, the pioneer of the MP3 file sharing movement, launched in 1999. Its demise came earlier this year following a high-profile lawsuit brought by the Recording Industry Association of America and opposition from bands such as Metallica.