Napster is appealing against Wednesday's ruling that it suspend trading until its copyright-infringement prevention filters can be shown to work.

Napster interim CEO Hank Barry said the order - issued on Wednesday by judge Marilyn Hall Patel - is at "odds with February's Appeals Court ruling, and is a threat to all peer-to-peer file sharing".

The February ruling described as "too broad" a previous ruling by the District Court that would effectively have shut down the service. The case was then kicked back down to the lower court to be modified. In March, Patel re-crafted her injunction, allowing Napster to remain operational, as long as it removed copyrighted songs on its service.

On Wednesday, Patel ruled that Napster, which has been down since June 1 while it installs new filtering software, must remain idle until its file identification software is shown to be 100 per cent effective in preventing copyright material from being shared. In addition, the court delegated a technical expert to further examine Napster's system, the company said.

In the statement, Barry said that at the time of the hearing the company's filters were "over 99 per cent" effective, but confirmed that Napster will obey the ruling.

While the statement said that Napster will file its appeal on an "expedited basis", it also added that it will "explore other options for resuming transfers as soon as possible".

Napster added that it expects to launch its subscription-based service later in the third quarter.