Katy gave evidence in the week-long trial, denying she had ever heard the 2009 song Joyful Noise by Flame before recording her track.

She even offered to perform Dark Horse for the court room when her lawyers were unable to play the song for jurors because of a broken speaker system.

But despite the joke, the jury ruled against Katy on Monday.

During the trial, Katy's team described the beat of both songs as "commonplace", arguing that because of that, Flame (real name Marcus Tyrone Gray) could not claim copyright.

"They're trying to own basic building blocks of music, the alphabet of music that should be available to everyone," said Katy's lawyer Christine Lepera during her closing arguments in court last week.

But lawyers representing Flame claimed that Katy and her team had "copied an important part" of his song, at the conclusion of legal proceedings that started in 2014.

"They're trying to shove Mr Gray into some gospel music alleyway that no one ever visits," said Gray's lawyer Michael A. Kahn.

He also mentioned the fact that Katy started her career as a Christian artist.

Despite the trial focusing on the production of the song - it was produced by Dr Luke, Max Martin and Cirkut - the court found that the six songwriters of Dark Horse were also liable.

This includes Katy Perry, and rapper Juicy J, who contributed a verse to the song.

Katy and Juicy J at the 2014 MTV Video Music Awards, where Dark Horse won best female video

Today (Tuesday 30 July) the court will start work on deciding how much in damages Gray may be owed.

Dark Horse was released in 2013 on Katy's fourth album Prism and is one of her biggest hits.

It has sold more than 13 million copies worldwide and the video for the song was the first ever by a female artist to reach a billion views on both YouTube and Vevo.

In total, the video has been watched more than 2.6 billion times since its release in 2014.