NEW YORK (Reuters) - Online retailer Amazon.com Inc said on Thursday its digital music store will now offer songs from Sony BMG Music Entertainment without copy-protection technology, or digital rights management.

Amazon said the deal makes it the first retailer to offer customers DRM-free songs from all four major music companies in the MP3 format.

Songs in MP3 format can play on the widest range of digital music players including Apple Inc's iPod, Microsoft Corp's Zune and various mobile phones.

Sony BMG, which is the second largest music company in the world, is home to such artists as Beyonce, Britney Spears and Celine Dion.

Earlier this week, Sony BMG became the last of the four major music companies to start selling its digital songs without copy protection with the launch of its MusicPass service.

The music industry posted a 15 percent drop in U.S. album sales in 2007 as fans bought fewer CDs. Though digital music sales have been rising, they have not made up for the revenue shortfall, forcing executives to consider new business models and methods to boost sales.

One of the issues for music companies in 2007 was whether dropping DRM protection would help drive digital music sales.

Fans have been frustrated by the limitations imposed by DRM, which can prevent a user from playing a digital song on an incompatible PC or portable media player.

Music companies had originally required digital music retailers use DRM to prevent customers from making multiple copies or sharing songs with friends for free.

EMI, the number four music company in market share, became the first major company to drop DRM in April. It was soon followed by Vivendi's Universal Music Group, which ranks No. 1, and Warner Music Group.

Sony BMG is jointly owned by Sony Corp and German media group Bertelsmann AG (BERT.UL).