Backed into a corner by the Recording Industry Association of America, Napster finally landed a telling blow, accusing its rivals of seeking to set up an online music monopoly.

The internet firm, bidding to avoid a summary judgement that would take the case straight to deciding what penalties Napster must pay, argued a number of points in a hearing before Judge Miriam Patel earlier this week.

Although critical of Napster to date, US reports say those close to the case believe Judge Patel was sufficiently swayed by its antitrust allegations to concentrate on questioning the big music publisher's joint ventures, MusicNet and PressPlay (formerly Duet).

The services are already the subject of a Department of Justice investigation, and enquiries by the European Commission.

MusicNet is owned by Warner, EMI, Bertlesmann, Zomba and streaming media firm Real Networks. PressPlay is backed by Sony and Universal.

Experts said that if Napster won this fight, Judge Patel was almost certain to order changes to the two services.