It's been nearly a year in the making but Spotify has reportedly struck a deal with Sony, Universal and Warner that will allow them to offer a free version of their streaming music service for mobile devices. An announcement on the matter is expected at a Spotify press event on December 11 in New York City according to people familiar with the matter as reported by the Wall Street Journal.

At present, Spotify only allows non-paying members to listen to tunes through their desktop client. Moving forward, however, we are told that Spotify will allow users to play a limited selection of songs on demand over mobile. The service will mostly be used to stream music based on the user's input, similar to how Internet radio services like Pandora work.

Many are probably asking why it took so long for Spotify to introduce a free model over mobile in the first place. The answer has to do with the major record companies and a dispute over the rates Spotify would pay to play songs over mobile to non-paying subscribers as well as how much control these members would have over the music they listen to.

Naturally, the new service will be ad-supported but that's pretty much a given in this day and age of advertising and free services. But with any luck, Spotify will be able to convert more free users into paid users. At present, the service enjoys 20 million active users worldwide although only 6 million are paying subscribers.