Illegally downloading just a single blockbuster movie or hit song using so-called "torrent" software means your computer is likely to be secretly logged and monitored by copyright enforcement agencies, undercover research has shown.

The digital details of filesharers using websites such as Pirate Bay are monitored within just three hours of downloading popular films, music, e-books or software programs, having their computer's unique IP address stored as "first-hand evidence", according to a three-year study by the University of Birmingham.

Obtaining a pirate version of the latest Batman film is far more likely to see your computer activity spied on than if you hunt down an obscure Italian post-modernist arthouse flick, however. The study, led by Dr Tom Chothia, found that "monitoring is prevalent for popular content… but absent for less popular content".

Users of BitTorrent, the advanced peer-to-peer file-sharing system that allows people to download files in fragments from "swarms" of multiple users rather than directly from one source, may have been monitored for as long as three years.

A team of computer scientists made the discovery by building their own software that acted like a BitTorrent file-sharing client and tracking all the connections that were made to it.

Ten monitoring companies were identified by the research, but it is unclear what they do – or plan to do – with the information. There has been speculation that the details may be stored in anticipation of a new and more intense crackdown on filesharers in the courts.

However, the paper noted that "direct monitoring, in its current form, falls short of providing conclusive evidence of copyright infringement". Some of the monitoring may also be carried out simply for research into the extent of filesharing.

Given the fact that just one download is enough to attract a monitoring firm to the culprit's details, the sheer number of filesharers whose Internet activity has attracted their attention will be huge.