Albums by artists such as Avril Lavigne will be available DRM-free

The decision means that all the world's four biggest music companies offer music free of software locks, or Digital Rights Management systems.

Music fans will be able to get at the DRM-free tracks by buying gift cards for Sony's Platinum MusicPass service.

Sony BMG said the DRM-free MP3s would go on sale in the US from 15 January.

File sharing

Those buying the gift cards will be able to download albums from the MusicPass.com website. At launch 37 albums from artists such as Celine Dion, Avril Lavigne and Bruce Springsteen will be available.

Customers will be able to get access to the downloadable tracks by typing in an identifying number on the back of the gift card. Cards are expected to cost $12.99 (�6.58) and will be available from stores such as Best Buy and Target.

The service will launch in Canada in late January but Sony has not announced plans to launch it in Europe.

"The MP3 files delivered through MusicPass play on computers, as well as on all MP3 players, including iPods," said Thomas Hesse, president of SONY BMG US, in a statement.

Sony is the last of the big four record labels to launch music services with tracks free of digital locks that limit what people can do with the music they buy.

Growing trend

In April 2007 EMI announced that every track it made would be available in a DRM-free format. Apple's iTunes store was the first to offer copies of these tracks to customers.

Less ambitious trials of DRM-free tracks have been tried by Vivendi Universal and Warner Brothers.

Vivendi Universal started selling tracks free of copy protection via the Wal-Mart and Amazon online music stores in January. Warner is also offering its DRM-free MP3s via Amazon's store.

The digital locks were put on the music tracks in a bid to stop people pirating the music and swapping it on file-sharing networks.

The different online stores selling music also used different types of DRM to further limiting what people could do with the tracks they bought.

By dropping the DRM music companies are hoping they can boost music sales.