Megan Fox, who made her name as Shia LaBeouf's love interest in the first two Transformers films, will no longer appear in the franchise.
Fox played Mikaela Banes in the 2007 blockbuster and sequel, which together made $1.5bn (£1.05bn) worldwide.
Production is due to start this month on the third instalment, with a release date set for July 2011.
Paramount Pictures declined to give further details or say who would replace the 24-year-old actress.
But Fox's publicist told People.com that it was the actress's decision not to play a part in the third film.
"Megan Fox will not be starring in Transformers 3," the publicist said.
"It was her decision not to return. She wishes the franchise the best."
Fox's absence from the franchise follows a public spat between her and Transformers producer Michael Bay last year, in which she reportedly said he was "a nightmare" to work for.
But Bay later defended the actress and said he looked forward to working with her on part three.
In announcing Transformers 3 on his website in October, Bay wrote: "P.S. Megan Fox, welcome back. I promise no alien robots will harm you in any way during the production of this motion picture".
Last year, Fox starred as the title character in Jennifer's Body, written by Juno's Diablo Cody, playing a possessed cheerleader who preys on high school boys.
Her latest film sees Fox star opposite Josh Brolin and John Malkovich in the supernatural Western Jonah Hex, which is due for release next month.