SEATTLE — Warner Music Group, a major holdout on selling music online without copy protection, caved in to the growing trend yesterday and agreed to sell its tunes on Amazon.com Inc.’s digital music store.

Until now, Warner Music had resisted offering songs by its artists in the MP3 format, which can be copied to multiple computers and burned onto CDs without restriction and played on most PCs and digital media players, including Apple Inc.’s iPod and Microsoft Corp.’s Zune.

The deal raises the total number of MP3s for sale through Amazon’s music download store to more than 2.9 million. Warner Music’s entire catalog, including work by artists Led Zeppelin, Aretha Franklin, and Sean Paul, will be added to the site throughout the week. The Amazon store launched with nearly 2.3 million songs in September.

Major music labels Universal Music Group and EMI Music Group Plc had already signed to sell large portions of their catalogs on Amazon, as had thousands of independent labels. Most songs cost 89 cents to 99 cents each and most albums sell for $5.99 to $9.99.

The chairman and chief executive officer of Warner Music, Edgar Bronfman, Jr., had been reluctant to follow in the steps of the rival recording companies.

In an e-mail obtained by the Associated Press and distributed to Warner employees yesterday, Mr. Bronfman noted that selling downloads without DRM would help spur new types of online music applications and foster competition among online retailers.