In an interview with The Daily Beast, Monjack said he is planning to file a wrongful-death action against the studio for allegedly firing the actress from "Happy Feet 2" two weeks before her death.
Monjack claims she suffered a heart attack as a result of stress related to being let go from the project.
"They killed her," he told The Daily Beast. "She was devastated."
The Los Angeles County Coroner's Office has not released an official cause of death for the 32-year-old actress, who died on December 20 in her LA home.
Monjack also went on to claim that his late wife was not taking illegal drugs.
"In all the time I've known her, she has never, and I repeat never, done drugs," Monjack told The Daily Beast. "Not a line of cocaine, not a hit from a joint, nothing. She was anti-drugs. There are no drugs involved."
Monjack claims the only prescription drug that his wife took was Klonepam, an anti-anxiety medication prescribed to control the seizures she occasionally suffered from.
"The drug rumors made her lose roles, I'm sure," Monjack said. "All she wanted to do was to make movies. She was waiting for the role that would revive her career, waiting for the call from Penny Marshall or Gary Fleder, people she had worked with before, that they might remember how talented an actress she was and call with a new magical role."
Access Hollywood has reached out to Warner Brothers for comment and is awaiting a response.
Related Content from AccessHollywood.com:
VIEW THE PHOTOS: Brittany Murphy Remembered
PLAY IT NOW: Brittany Murphy's Husband Addresses Drug Rumors
PLAY IT NOW: Access Archives: Brittany Murphy Is An Access 'Producer For A Day' (July 9, 2004)