Apple faces a law suit claiming the tie-up between iPod music players and music bought at its iTunes online music store violates US anti-trust laws.

The suit was filed by a user, Melanie Tucker, back in July but was revealed only yesterday in the company's annual report, which was delayed by another legal tussle over the Steve Jobs stock options affair.

It claims that Apple illegally prevents music bought from iTunes to be played on rival music players, and does not make clear to customers that this is the case, according to US reports.

And it calls for a ban on the tie-in and damages for anyone who has bought an iPod or iTunes music track.

Apple claims in a court filing that innovation would be stifled if interoperability were stifled and that the "anti-trust laws, in short, require companies to compete, not co-operate with each other". But a judge has already implcitly rejected the argument by refusing to dismiss the suit.

The case, which ironically mirrors some issues central to Microsoft anti-trust actions, follows similar cases by consumer groups in Europe. Chief executive Steve Jobs dismissed as " state-sponsored piracy" a French law that would forbid the iTunes tie-in.