What IFPI claims was the largest-ever operation to tackle internet piracy in Bulgaria has resulted in the closure of four websites suspected of illegally distributing music, films, games and software on a massive scale.

Bulgarian web sites nanoset.net, rapidadd.com, 4storing.com and afasta.com have all been shuttered with immediate effect following an investigation, which was coordinated by the Bulgarian Police's Cyber Crime Unit.

The operation is part of an on-going anti-piracy strategy of the Bulgarian Culture and Interior Ministries. The IFPI-affiliated Bulgarian Association of Music Producers (BAMP) filed the original complaint against the illegal services, which it says were run by an organized criminal network.

Of the four sites shuttered by Bulgarian authorities, nanoset was a P2P platform, while other three sites were file-hosting and storing sites, offering both downloads and streaming of unlicensed music and film content.

Police believe users had paid an approximate total of BGN 5 million (€2.5 million) to access unlicensed repertoire via the websites during the time that they were operational. Revenue was also raised through advertising sales.

As part of the investigation officers made several arrests and seized documents and computer systems from the suspected operators.

Investigators searching the premises of Bulgarian ISPs being used by the illegal websites also discovered servers containing more than 120 terabytes of unlicensed content, the equivalent of more than 200,000 CD-Rs.

According to IFPI, Bulgaria has the lowest revenues from physical format sales in Europe. In 2009, that totalled $3.6 million. The country also generates negligible income from digital sales ($700,000 in 2009) and has a piracy rate estimated at almost 100%, with three of the five most-visited domestic web sites, excluding international sites such as Google or Facebook, being infringing services, according to international web information company Alexa.

In a statement, BAMP executive director of Ina Kileva said: "The sheer scale of this operation, the quantity of seized pirate content and the vast financial profit of the criminals involved shows clearly what an enormous problem piracy is in Bulgaria."

"It is depriving creators and artists of their livelihoods, feeding organized crime and frustrating all efforts to build a successful legal digital music market," Kileva went on to say, adding: "it is only by such actions that Bulgaria can hope to develop a legitimate digital economy in the future."