Katy Perry has had the result of a long-running copyright battle over the song Dark Horse overturned in her favour.

Seven months after a California jury decided that Perry, producer Lukasz Gottwald, and others had plagiarised Christian rapper Marcus Gray's track Joyful Noise, US District Court Judge Christina Snyder has reversed the verdict.

Last summer Gray, aka Flame, was awarded a $2.8 million payout at the jury trial, with Perry herself liable for $550,000 of the total.

"It is undisputed in this case, even viewing the evidence in the light most favourable to plaintiffs, that the signature elements of the 8-note ostinato in Joyful Noise . . . is not a particularly unique or rare combination," Judge Christina Snyder wrote in a ruling.

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"Because the sole musical phrase that plaintiffs claim infringement upon is not protectable expression, the extrinsic test is not satisfied, and plaintiffs' infringement claim - even with the evidence construed in plaintiffs' favour - fails as a matter of law."

During the initial trial, Perry's lawyer Christine Lepera had argued Gray and his team were "trying to own basic building blocks of music, the alphabet of music that should be available to everyone".

This decision comes just a week after the Ninth Circuit Court of Appeals handed Led Zeppelin a win in another important copyright case around rock classic Stairway to Heaven.

That ruling determined that the song did not infringe on Spirit star Randy Wolfe's song Taurus and brought a six-year legal row to an end.