Amazon, the world’s largest online retailer, has launched its long-awaited challenge to Apple’s digital music business with a new, aggressively priced downloading service that will be available to users of Apple’s popular iPod players.

Amazon is the largest online seller of physical CDs. Its new downloads will be “DRM free”, meaning that they can be accessed by both iPod and MP3 players, and be easily copied or burnt on to users’ CDs.