Pregnant pop star Madonna has found herself in the midst of a Net piracy wrangle after an unreleased single was leaked on the Web.

The single, to be Madonna's next single and the title track of her new album, Music, was due for a September or October release. But the song was apparently stolen, and large chunks can now be found via Napster software or on several fan sites.

Madonna, her manager and record label, Warner, are all said to be fuming, and threats against sites found offering the download were flying around yesterday.

In a statement, Warner warned: "Warner Bros regards the copyright infringement of its artists' recordings, including the unauthorised use of such recordings on Internet sites, as an extremely serious matter.

"Any site that posts or 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."

Meanwhile, Madonna's manager in the US, Caresse Norman, confirmed the music had been stolen. "[It] was not intended for release for several months. It is still a work in progress," he said.

"Ultimately those sites that offered a download of Madonna's music are violating her rights as an artist."

Madonna is not the only artist pulling her hair out over Napster – rock band Metallica and rapper Dr Dre have both set their legal eagles on the company. ®