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Those ads will point to gBox, a Californian start-up selling tunes for the standard 99 cents. Google will take its usual fee for running the ads and will continue to operate its music search service, which directs users to a selection of online music stores when they search for specific albums or artists.

Google reaffirmed that it has no plans to open its own online music store.

gBox meanwhile will hope that search-based advertising is an effective way of drumming up demand for digital music. Its unrestricted Universal tracks will be encoded as MP3s so they will play just about anywhere.

The same songs - from a selection of Universal's artists - will also be sold by Amazon, one it has its long-awaited digital music service up-and-running. Apple, on the other hand, will have to content itself with selling unrestricted EMI tracks through iTunes while providing Universal with a control group for measuring the impact on sales and on file sharing of DRM-free tunes sold elsewhere.