By Aidan Malley

Published: 07:05 PM EST

Landing what may be one of the most significant blows yet against Apple's iTunes, Amazon on Thursday said it would carry Sony BMG's music -- providing the download service with unprotected music from all four major music labels while Apple claims only one.

The deal confirms a magazine leak and sees Sony BMG joining EMI, Universal, and Warner in the roster of music providers who sell albums in MP3 format through the Amazon store.

As with the rest of Amazon's catalog, songs from Beyonce and other Sony artists will be copyable an unlimited number of times and can be played with virtually any computer software or handheld audio device, including Apple's iTunes jukebox and the iPod lineup.

The move creates a conspicuous imbalance between Apple and Amazon in terms of their ability to offer music without copy protection, a feature many have considered essential to encouraging more sales of direct-download music while CD sales drop. iTunes was the first to obtain major-label music without digital rights management (DRM) from EMI in spring 2007 but quickly saw its advantage fade as Universal and Warner joined EMI in signing on to Amazon's digital store in the remaining months of last year.

For a third time, the announcement also comes without word of similar deals for competing stores, including current market leader Apple. Universal has back out of long-term deals to provide music to iTunes after complaining about inflexible pricing at the Apple-run store but is commonly believed to have signed with the even lower-cost Amazon MP3 as a way to gauge whether it can reduce Apple's influence on digital music sales.

Amazon's coup further corroborates alleged insider tips from the music chart keepers at Billboard, who reported late last year that Sony BMG would sign with Amazon and participate in a Superbowl promo which would give away as many as one billion free Amazon MP3 song codes hidden underneath Pepsi bottle caps.