Pirate Bay founders Gottfrid Svartholm Warg and Peter Sunde at their trial at Stockholm's city court. A verdict in the case over their role in file-sharing copyrighted materials is due on April 17, 2009. ((Scanpix Sweden/Bertil Ericson/Associated Press))

Internet traffic in Sweden fell 33 per cent on the day the country's new anti-piracy law came into effect, according to Swedish online measurement firm Netnod.

The internet tracking firm reported traffic fell from an average of 120 gigabits per second to 80 gbps on Wednesday, the day a new law, based on the European Union's Intellectual Property Rights Enforcement Directive (IPRED), came into effect.

The new law requires internet service providers to provide subscribers' Internet Protocol (IP) addresses to copyright holders in cases where a court finds sufficient evidence of illegal activity.

Sweden has become a battleground between online consumers of digital media and the entertainment industry over the practice of peer-to-peer file sharing.

About eight per cent of the country's population engages in peer-to-peer file sharing, according to the government statistics agency.

Pirate Bay, the popular BitTorrent sharing site, is also located in Sweden.

BitTorrent is an internet protocol that allows distribution of files between computers on the net without a central server. When users find a file on a BitTorrent network, they click on it and connect to another computer, called a seeder, that has that file and the download begins.

It's this arms-length approach that has allowed Pirate Bay to avoid the same fate as file-sharing sites such as Napster, which actually distributed copyrighted material.

Netnod's figures, which measure traffic in and out of country, showed steady result the week before the law came into effect, suggesting its introduction may have induced at least a temporary chill on internet use.

A similar drop in traffic also occurred when Swedish authorities raided the Pirate Bay offices in 2006, according to information technology newspaper Computer Sweden.

Pirate Bay estimates about 22 million people use its site, about a third of them in China.

Pirate Bay's four founders are currently in a legal battle with movie studios Warner Bros., MGM, Columbia Pictures, 20th Century Fox and Sony, who allege the website is facilitating the theft of their movies and are suing for $14.3 million US.

A decision in that case is due April 17.