The war of words between the record industry and Napster intensified today when the Recording Industry Association of America hit out at the song-swapping service.

It branded Napster's attempts to block copyrighted songs an "utter failure" and upped the pressure on courts to shut down the internet service by filing a new court order for Napster to change the filter software it is using to charge users to download tracks.

The RIAA is furious because it says users are still able to download tracks for nothing.

"Napster seems to have adopted the most porous filter available. It's not working, it never will work and Napster should be ordered to implement an effective filter or to change its filtering method," said Hilary Rosen, president of the RIAA.

The association also maintains that many of the songs Napster claims to have filtered are still available on the site.

A bewildered Napster hit back by saying it was doing the best it could and had so far managed to block 275,000 songs.

It defended itself by saying that the record companies had not co-operated sufficiently to allow it to comply with the original court order from three weeks ago.

Napster has also had problems identifying song titles that users had deliberately misspelt to avoid filters.

Napster's chief executive Hank Barry stressed that "Napster is aggressively complying with the injunction with significant measurable results".

However, pressure is growing on Napster, which is due back in court on April 10.

In its court filing, RIAA said it had searched for the 212 copyrighted works it originally asked Napster to block in its lawsuit and found every song still available on the site.

Napster, however, is determined not to allow legal red tape to get the better of it. It is encouraging hard-core fans to rally round and descend on Washington next Tuesday when senators meet to discuss the legal battle.

But with usage down by a quarter since it brought in controls, Napster seems to be gradually losing its once unequivocal support.

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