The Norwegian government has said that iTunes, Apple's online music store, is illegal, and that the company must open access to its music downloads by October or face legal action.

In June, Norway's ombudsman upheld a complaint by the country's consumer council that by forcing customers to play their downloaded music only on Apple's iPod, iTunes violated Norwegian law - a decision which prompted other European countries to review the situation.

Today the ombudsman, Bjoern Erik Thon, said that Apple must make iTunes music compatible with other players than the iPod by the end of September - "or we will take them to court."

"iTunes is imposing unreasonable and unbalanced restrictions that are not in accordance with Norwegian law," Mr Thon said.

He said that Norwegian courts could impose fines on Apple until songs downloaded from iTunes were playable on rival devices.

The case would be heard firstly by Norway's Market Council, a legal body which can demand changes to contract clauses that are found to be unreasonable, and if appealed, it would be heard in the ordinary courts.

Apple has faced pressure in several European countries over iTunes, with consumer rights groups organisations in Germany, France, Finland and Norway this week agreeing a position in their struggle against the company.

They say that the digital rights technology Apple uses inhibits consumers' free use of the songs they buy on iTunes, including the ability to copy and transfer songs to devices other than the iPod.

"We believe consumers have a right to play material purchased online on a portable device of their own choice. Contract clauses that make this impossible or too inconvenient are unfair and should be revoked," the organizations said in a statement.

"Apple is aware of the concerns we've heard from several agencies in Europe and we're looking forward to resolving these issues as quickly as possible," an Apple spokesperson said earlier in the week.

Which?, the UK consumer rights body, said today it had not yet taken a position over interoperability. No comment was immediately available from Apple.