Ed Sheeran is being sued for more than £13 million for allegedly copying a song released by X Factor winner Matt Cardle.
A 20 million dollar lawsuit filed in a Los Angeles federal court claims Sheeran's 2014 summer hit Photograph infringes the copyright of Cardle's 2012 single Amazing.
Lawyer Richard Busch, who recently triumphed on behalf of Marvin Gaye's family in a copyright case over the Robin Thicke hit Blurred Lines, is representing Amazing's songwriters Martin Harrington and Thomas Leonard.
The songs' similarities "reach the very essence of the work", the lawsuit reads.
"The similarities go beyond substantial, which is itself sufficient to establish copyright infringement, and are in fact striking. The similarity of words, vocal style, vocal melody, melody and rhythm are clear indicators, among other things, that Photograph copies Amazing."
According to court papers posted online, the lawsuit alleges that the choruses of Amazing and Photograph share 39 identical notes.
Sheeran will have to answer the charges alongside his Photograph co-writer, Snow Patrol's Johnny McDaid, as well as other co-defendants including Sony/ATV Songs, Warner Music and Polar Patrol Music Publishing.
Cardle, who won the ITV talent show in 2010, is not a party to the lawsuit.
The plaintiffs claim that the team behind Photograph were aware of Cardle's single.
The lawsuit states: "Given the striking similarity between the chorus of Amazing and Photograph, (the) defendants knew when writing, publishing, recording, releasing, and distributing Photograph that they were infringing on a pre-existing musical composition.
"The conduct of (the) defendants has been wilful from the inception of the creation of Photograph."
It adds: "The copying of Amazing by Photograph is breathtaking in its deliberateness, magnitude, and hubris."
Parts of both songs are transcribed in the court papers to demonstrate alleged similarities.
The parallels are also said to be "instantly recognisable to the ordinary observer", with the songwriters saying the alleged copying makes up "nearly one half" of Sheeran's single.
The Brit Award-nominated single is also featured on the soundtrack for new movie Me Before You.
Country music star Keith Urban has previously said he fell in love with Photograph at Wembley Stadium watching "everybody in the audience sing along".
Harrington and Leonard claim that while Sheeran and his co-defendants have received "career-defining accolades, awards, and a fortune" for Photograph, they have received nothing.
The suit states: "Little did Mr Urban or the crowd know, they were singing along to the Amazing chorus written by Harrington and Leonard.
"Being acknowledged by Mr Urban and others of his stature, and having music highlighted in a major motion picture, are game-changers for the careers of active songwriters.
"Songwriters are deprived of these opportunities when music is wrongly taken from them, as has happened here."
The lawsuit states that the defendants had already been given notice of the complaint, but it had not been resolved before papers were filed at court.
The songwriters have asked to be awarded the profits on the song, which are believed to be more than 20 million dollars.
A permanent injunction is also sought on Photograph, which could prohibit Sheeran from performing the song in future or selling recordings.
If this is not possible, the plaintiffs are seeking royalties.
Last year, a jury determined that Robin Thicke and Pharrell Williams had copied Marvin Gaye's 1977 hit Got To Give It Up.
Gaye's children were awarded nearly £4.9 million, after a trial which had focused on detailed analyses of chords and notes in Blurred Lines and Gaye's song.