A sometimes-riled Ed Sheeran on Monday denied copying Marvin Gaye's classic "Let's Get it On" in his second day of testifying to jurors in a copyright trial in Manhattan federal court, calling the claim "insulting."

As he had last week, the British singer-songwriter played guitar and sang from the witness stand to underscore his testimony, telling jurors his song "Thinking Out Loud" had actually been inspired by Irish musician Van Morrison.

"I find it really insulting to devote my whole life to being a performer and a songwriter and have someone diminish it," said Sheeran, who is being sued by heirs of Gaye's co-writer Ed Townsend.

Lawyers for the heirs last week displayed a video of Sheeran transitioning seamlessly between "Thinking Out Loud" and "Let's Get it On" in a live performance they said amounted to a confession that he had ripped off the song.

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Sheeran said he and other performers frequently perform such "mash ups," and that he had on other occasions combined "Thinking Out Loud" with Van Morrison's "Crazy Love" and Dolly Parton's "I Will Always Love You."

He also said that an expert witness for the heirs, in transcribing his song, had altered it to make the chords and melody more like "Let's Get It On."

"I don't know why he's allowed to be an expert," he said.

Sheeran became combative under cross-examination by Patrick Frank, a lawyer for the heirs. He ridiculed Frank's questions about how often Sheeran collaborates with others in writing songs, which he said was common practice.

"You're not, like, breaking new ground here," he said.

Sheeran's co-writer Amy Wadge took the stand after his testimony concluded, telling jurors about their friendship and collaboration of 17 years. She also denied copying "Let's Get It On" and pointed to other songs with the same chords as "Thinking Out Loud," including Shania Twain's "You're Still The One."

If Sheeran is found liable, there will be a second trial to determine the damages amount.