Pop star Madonna has entered into the battle against Napster following the leak of an unreleased single on the internet.

The single, Music, to be the first from her forthcoming album of the same name, was due to be released in September or October. Most of the song can now be accessed by Napster software or on fan web sites.

The Napster software, created last year by Shawn Fanning, aged 19, allows MP3 users to access a directory of songs stored on the hard drives of all its members, giving each member near-instant access to hundreds of thousands of songs, many of them illegally copied.

Madonna’s record company has threatened legal action against web sites offering the download. Warner Bros said in a statement: “Any site that makes available our copyrighted material without our consent runs the risk of civil and criminal prosecution under existing federal law and state common law. We expect that the site owners who have included this material on their site will comply with our legitimate request that they cease permitting unauthorised downloads of this song.”

There have been recent actions regarding Napster’s software by the rock band Metallica and rappers Dr. Dre and Eminem. In May, Metallica blocked more than 300,000 usernames from the Napster site.