The Love Actually star has penned the screenplay for a movie version of the lives of 19th century poet John Ruskin, his wife Effie Gray and her affair with pre-Raphaelite painter John Everett Millais.
The subject was successfully tackled by Gregory Murphy in his play The Countess, which was a hit on Broadway in 1999 and was taken to London's West End in 2005.
But according to the Hollywood Reporter, Murphy is concerned Thompson's script infringes on his own work, and the film's producers have taken the case to a New York federal court.
A lawsuit filed by the producers claims Thompson had no access to Murphy's play and did not copy it. They want a declaration from the judge stating that there are no copyright issues so the film's financing can move forward.
The papers state, "In order to close financing to produce a motion picture based on Effie, (the plaintiff) must be able to demonstrate that there is no validity to Mr. Murphy's claim of infringement."
Orlando Bloom, Saoirse Ronan and Thomson are all set to star in the movie.