The world's largest music label Universal Music Group is experimenting with the sale of DRM-free downloads - but not through iTunes.

Universal artists include Amy Winehouse and 50 Cent. The company plans to make tracks available free of digital restriction through selected services, including Google, Wal-Mart and Amazon.

Universal says it will run an initial scheme between August and January in order to get information about demand, price sensitivity and any potential piracy problems.

The label says it is not selling tracks DRM-free through iTunes in order to use that service as a "control group".

But Universal last month refused to agree a new long-term contract to sell music through iTunes.

It's thought that Universal is attempting to push back against the growing influence of iTunes, perhaps by attempting to shore up use of other digital music services.

Apple is presently the third largest music retailer in the US.