The number of people visiting Napster is declining to such an extent that the company has now issued an apology for "overblocking" access to music files.

Napster has been implementing a filtering system to block copyrighted material from its users, in an effort to comply with the court injunction ordered last month in the US.

Napster admitted that in implementing a range of filters, to block access to songs that are copyright protected, it has unintentionally removed tracks that can be shared legally. The "overblocking" was unintentional, and the company plans to refine its filters to "avoid overblocking to any extent possible", Napster said.

Names change Napster warns users against trying to circumvent the filters by giving the files new names. "Napster's terms of service prohibit the use of evasive measures such as pig Latin, napcameback, napsterdecoder and otherwise deliberately altering file names in order to evade Napster's filters."

Napster remains one of the most popular Web sites, but has lost one fifth of its users since the injunction was imposed, according to a survey published Thursday by Internet analyst Jupiter Media Metrix.

Napster's unique users fell from 15 million in February, to 12 million in March, according Andreas Gutjahr, Jupiter's European marketing manager. In February, Napster was still used by 14.3 per cent of online users at home in the world's 13 most wired countries, the Jupiter report revealed.