Napster has claimed a first with the development of a subscription portable music service that delivers legal audio content to next-generation mobile phones.

The division of Roxio has extended its Napster To Go service to let music fans transfer and play an unlimited number of full-length songs on smartphones.

The offering currently allows users of Microsoft Windows Media Player 10 to transfer content to MP3 players.

The subscription-based offering is initially only supported for the newly released AT&T Wireless-based Audiovox SMT5600 Smartphone. The handset also supports individual tracks purchased by Napster subscribers or downloaded from the Napster Light store.

The Napster To Go portable digital music subscription service, which was previewed in September 2004, works via a plug-in to Windows Media Player 10.

An introductory $14.95 per month lets consumers move an unlimited amount of music from Napster's library to compatible MP3 players and portable devices.

The firm promised that the service would "officially debut" later this year with the release of Napster 3.0.

"Napster was the first service to make the promise of the portable subscription model a reality, and today we are taking that innovation to the wireless marketplace," said Chris Gorog, Napster's chairman and chief executive.