A US judge has effectively overturned a ruling that would have seen Microsoft pay $1.5 billion for violating patents on MP3 players.

Ruling on post-trial motions in a lawsuit Alcatel-Lucent already won, San Diego Judge Rudi Brewster said one patent in the case wasn't infringed by Microsoft. On the other patent, he said the court didn't have jurisdiction because a co-owner of the patent didn't join in the suit.

In February, the jury awarded Alcatel-Lucent $1.5 billion in damages for infringement of two MP3 patents. It was one of six patent cases involving the two companies. Lucent Technologies had brought the suit in 2003 before it merged with Alcatel.

Yesterday, Judge Brewster ruled that Microsoft had not infringed one of the two patents. On the other patent, the judge found that Fraunhofer, a research organisation based in Europe, was a co-owner of the patent with Alcatel-Lucent. Because Fraunhofer didn't join in the suit, Brewster's court didn't have jurisdiction.

Microsoft already licenses the MP3 patents from Fraunhofer, according to Microsoft spokesman Guy Esnouf.

Alcatel-Lucent said it will appeal the rulings at the US Court of Appeals in Washington DC.

"This reversal of the judge's own pre-trial and post-trial rulings is shocking and disturbing, especially since - after a three-week trial and four days of careful deliberation - the jury unanimously agreed with us, and we believe their decision should stand," Alcatel-Lucent said in a statement.

The decisions remove the damage award, Microsoft's Esnouf said. Even if Alcatel-Lucent wins its appeal, both the rate of the royalty and the basis for calculating it - a percentage of the value of a PC - will be tried again, he said.