The legal fight between online song-swapping service Napster and the music industry is to return to court next month with both sides making increasingly acrimonious accusations.
In the latest round of the fight, the Recording Industry Association of America, which represents the leading record labels, has accused Napster of deliberately failing to comply with a court order to block access to copyrighted music.
Hilary Rosen, president of the association, said Napster was using outdated and ineffective technology to stop users downloading outlawed recordings. Tests by association members found that Napster was only stopping access to music if both the title and artist were searched for. So, for example, users could download Sting's Fields of Gold by searching for just the artist or the title of the song.
"Calling this type of filter effective is like calling an umbrella full of holes a hurricane shelter," said Ms Rosen. "It's not working, it never will work and Napster should be ordered to implement an effective filter or to change its filtering method."
The association, which filed its complaint three weeks after Napster was ordered to block access to copyrighted music, said there was a marked difference between the effectiveness of the company's technology for legal and illegal access.
"While Napster's filter works by blocking the precise artist and title, Napster's search engine intentionally finds and returns inexact matches."
After the latest court filing, Napster hit back by accusing the industry of acting in bad faith and failing to cooperate. Hank Barry, the company's chief executive, said Napster was "aggressively" complying with the injunction and had already blocked access to more than 275,000 songs and 1.6m unique file names.
"While we have gone forward to block this multitude of files, it is important to note that not a single record company has provided us with one variant of any song name," he said. "The RIAA's call for a fundamental change in Napster's technology is unsurprising. It is an attempt to change subject rather than cooperate with Napster as the injunction specifies."
He added that Napster had included more than 10,000 variations in artists' names and more than 40,000 variations in song titles in an attempt to block access. The number of files available through the Napster index has fallen by 57% from 370mto 160m. The average number of files being shared at the service has also dropped by almost two-thirds after the injunction on March 6.
The recording industry denied Napster's accusations and said that it had handed over details of 675,000 copyrighted works to be blocked. It called on the company either to use new fingerprint methodology to identify tracks or to use a "filter-in" service.
The latter would significantly change Napster's service as it would allow the company to offer only music it had certified as legal.
