Research shows that filtering out copyright-protected material has driven away 25 percent of Napster users.
A quarter of Napster's 60 million users have abandoned the song-swapping service since it brought in controls to prevent the illegal trading of copyrighted material, according to a survey carried out by research firm Webnoize.
The company has told Reuters that it had stopped more than 275,000 songs from being dowloaded.
Napster was ordered to introduce song filtering after losing a court case brought by representatives of the music industry including the Recording Industry Artists of America (RIAA).
Under the court ruling, record labels had to supply details of all their songs which they did not want shared on service and Napster, in turn, was ordered to remove them from its peer-to-peer file sharing service.
Napster has denied that the introduction of filters has driven users away. However, the RIAA claims that - nearly three weeks after the court ruling - the filters still haven't been fully implemented and accuses Napster of stalling.
Earlier research by Webnoize found that the introduction of filters has cut by half the average number of songs shared by a typical Napster user.
Representatives of Napster say the company is planning to lead a march on Washington D.C to coincide with a congressional hearing on online copyright issues.
