Warner Music has offered its catalogue without digital rights management (DRM) software for the first time.
John Reid, vice chairman of Warner Music Europe, said: "This deal will offer music fans a new level of flexibility in their use of tracks from our world-renowned artists.
"We believe that providing consumers with this assurance of interoperability will encourage sales of music downloads, and ultimately help the development of new digital music experiences.
"This agreement will enhance 7digital's service and benefit artists and music fans alike."
Music companies are increasingly looking at rivals like 7digital to break Apple's stranglehold.
"7digital is excited to be Warner Music's first major partner in Europe to bring MP3s to the marketplace," said Ben Drury, chief executive at 7digital.
"Our registered customer base has proved to us that MP3s sell very well and is the format of choice for digital media consumers."
The addition of Warner's MP3 catalogue means that over 80 per cent of 7digital's 3.5 million tracks are now DRM-free.
"7digital is committed to becoming the destination for MP3s and we plan to make our entire music catalogue available in DRM-free high-quality MP3 format by summer 2008," added Drury.
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