A courtroom in Stockholm is set to stage the internet piracy trial of the decade. Four men behind the hugely popular Pirate Bay filesharing website are about to go before a judge for enabling millions of internet users to make illegal downloads of music, movies, games and software.

The Pirate Bay site, based in Sweden, is the world's largest BitTorrent tracker and search engine with an estimated 25 million active users. BitTorrent is a data-sharing protocol that makes it easy for users to transfer large files. The torrent files, listed on the site, contain all the information needed to download film or music files from others who have often copied them without permission.

Four men connected to the site have been charged with facilitating the distribution of copyrighted material. They were charged after police raids in 2006 when officers searched their premises in Stockholm and seized servers. The four defendants, Fredrik Neij, Gottfrid Svartholm Warg, Peter Sunde Kolmisoppi and Carl Lundström, face up to two years in prison and a fine of 1.2 million kronor (£100,000). The trial is due to start on Monday.

Music and film companies have also brought a civil claim for compensation that is being heard with the criminal prosecution. They are claiming compensation of 2.2 million euros, in connection with a sample of 23 music works selected by the prosecutor. The film industry is claiming damages of 10.9 million euros, in connection with four films and one television series selected by the prosecutor.

The Pirate Bay has been in the sights of the music and film industries for several years as concern has grown over the level of illegal file-sharing. The four defendants have run the site since 2004 after it was set up a year earlier by the Swedish anti-copyright organisation Piratbyrån.

The website lists hundreds of thousands of torrent files, linking to content including big Hollywood films, music tracks from every major recording star and software from leading companies. The Pirate Bay has claimed that half of the world's torrents are found using its trackers. The site is free to use and is supported by advertising.

According to a geo tracker provided by the Pirate Bay, in one 24-hour period earlier this month, there were 3.3 million unique users in China (22.4 per cent). The US had 1.6 million users (11 per cent) and the UK 824,000 users (5.6 per cent).

Public prosecutor Hakan Roswall told Reuters in January 2008: "It's not merely a search engine. It's an active part of an action that aims at, and also leads to, making copyright protected material available.

"It's a classic example of accessory - to act as intermediary between people who commit crimes, whether it's in the physical or the virtual world."

Defenders of the four men have pointed out that no copyrighted material is stored on Pirate Bay's servers and no swapping of files actually takes place there. Pirate Bay locates file sharers on the internet and acts as a directory. The Pirate Bay's legal adviser, Mikael Viborg, has stated that because torrent files and trackers merely point to content, the site's activities are legal under Swedish law.

The music and film industries have instigated a policy of pursuing those who distribute copyrighted content. They hope that the case will set down a marker against illegal file-sharing and act as a deterrent to individual downloaders. Jo Oliver, general counsel of the International Federation of the Phonographic Industry, which represents record labels across the world, told The Times: "This is an extremely detailed case which will reveal the operations behind the Pirate Bay. While publicly these are internet freedom fighters, we are hoping that this case will show that these are people who are making money by violating other people's copyrights"

The prosecution is expected to present evidence that the four men have set up the Pirate Bay to make substantial profits from advertising and may have used offshore accounts to safeguard their money.

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