Amy Winehouse is one of Universal's artists

It will allow the sale of thousands of albums and tracks available in MP3-form without the protection, known as digital rights management (DRM).

Most major recording studios insist music sellers use DRM technology to curb online piracy.

Universal artists include 50 Cent, the Black Eyed Peas, and Amy Winehouse.

Universal said: "The experiment will run from August to January and analyze such factors as consumer demand, price sensitivity and piracy in regards to the availability of open MP3s."

Retailers including Google, Wal-Mart, and Amazon.com, will participate in the DRM-free trial, Universal said.

But participants do not include Apple iTunes online music store, the third largest music retailer in the US - although iTunes already offers a selection of music from EMI free of copy protection, albeit for a higher price.