Other defendants listed in the new claim include Sony/ATV Music Publishing, the Atlantic record label and Amy Padge - who co-wrote the song.
The heirs of Mr Townsend sued Ed in 2016, also claiming Thinking Out Loud copied Let's Get It On. Sheeran's song that is under fire, "Thinking Out Loud" was nominated for three Grammys in 2015 and won one of them: song of the year.
The new lawsuit picks up where the former stopped, claiming that several musical elements-melody, rhythms, etc. -originated in "Let's Get It On".
Sheeran hasn't responded to the lawsuit, and a representative didn't answer HuffPost's request for comment.
This is the second time Sheeran, 27, has been accused of stealing from Gaye's 1973 hit.
Ed Sheeran is on the receiving end of a monster $100 million lawsuit by a company claiming the singer ripped off a Marvin Gaye classic.
In 2016, Sheeran was also accused of ripping off a song called awesome for another one of his most notable works, Photograph.
Two years later, the children were awarded $7.4 million by a jury.