Ed Sheeran defended his song Shape of You against copyright infringement claims in London's High Court on Monday.
Songwriters Sami Chokri and Ross O'Donoghue claim his 2017 song infringes "lines and phrases" from their song Oh Why.
Representing the Oh Why songwriters, Andrew Sutcliffe QC described Ed Sheeran as a "magpie" who "borrows" ideas from other artists to use in his own songs.
The case questions whether Sheeran "makes things up as he goes along" in his songwriting, or includes "collection and development of ideas over time which reference and interpolate other artists," Sutcliffe told the court.
"Sometimes he will acknowledge it but sometimes he won't," the representative continued.
In the witness box, Sheeran responded to this comment by saying "if Mr Sutcliffe would have done his research," he would have known he has cleared parts of songs with "lots" of artists in the past. Sheeran argued he was not aware of the song Oh Why while writing Shape of You.
Under cross-examination from Sutcliffe, he was asked how he could not be aware of Chokri as they were both regulars on the YouTube channel SBTV at the same time, Chokri sent messages to him on Twitter, and they met at a party.
"This is all stuff you're saying, this isn't stuff that's true," Sheeran replied.
Sheeran and co-writers Steven McCutcheon and John McDaid previously asked the High Court to declare they had not infringed Chokri and O'Donoghue's copyright to avoid a trial but were unsuccessful.
Monday marks the second day of the hearing, which is expected to last three weeks.
