Ed Sheeran (File Photo: AP)
"Shape of You", released 2017, was a huge hit for Sheeran, 31, and remains the most streamed-song ever on Spotify, with more than three billion streams.
It won Sheeran a Grammy for Best Pop Solo Performance. He has a writing credit on the track along with several others.
But two other songwriters, Sami Chokri and Ross O'Donoghue, allege that Sheeran's song has musical similarities to one they wrote called "Oh Why".
Sheeran, who turned up at the hearing in a dark suit and tie, denies the allegation
In 2018 Sheeran and the song's other credited writers launched legal action against Chokri and McDaid, prompting the pair to launch their own claim for "copyright infringements, damages and an account of the profit in relation to the alleged infringement".
The legal battle is expected to last three weeks, with judge Antony Zacaroli listening to both songs in court Friday.
Lawyer Andrew Sutcliffe, representing the aggrieved songwriters, told the judge that "the similarity between the two hooks is striking" and the songs "sound almost identical".
"This of course does not by itself prove that copying has taken place but it's a vital starting point," he added.
Sutcliffe suggested that Sheeran is a "magpie" who "borrows ideas" and will sometimes not acknowledge them.
Sheeran's lawyers have told the High Court that he and his co-writers have no memory of having heard the song "Oh Why" at the time.
The PRS for Music, which pays out royalties for the use of music, has temporarily halted royalty payments.