His autobiography was originally billed as "a candid look at the highs and lows of his life and career".
A spokeswoman for the actor declined to comment on his reasons for cancelling the book, which was due to come out this year. Robert, who has a history of drug abuse, signed the deal in 2006.
The actor is set to play the lead role in director Guy Ritchie’s forthcoming film version of Sherlock Holmes. It follows his stint as Tony Stark, a billionaire playboy who builds himself a superhero suit, in summer blockbuster Iron Man.
The actor signed on for three Iron Man films, and is due to reprise the role on the big screen in 2010.
Downey Jr has previously struggled with a series of personal problems, notably his widely-reported drug use, which led to prison and a rehabilitation programme. Source