Matthew Rhys has been cast in the title role in the upcoming HBO drama Perry Mason.

Executive producer Robert Downey Jr revealed on Twitter that The Americans star, fresh from his Emmy and Critics' Choice award wins, will lead the limited series.

The Iron Man star posted a picture of the script and wrote: "Now it seems to me the place to start is at the beginning.

"And it seems to me the beginning of any great project is casting my new best friend Matthew Rhys."

Based on the characters created by author Erle Stanley Gardner, the show will be set in Los Angeles in the early 1930s and will tell the backstory of the famous fictional criminal defense lawyer. Variety reports that the series will focus on a child kidnapping.

The original series first aired in the US in the 1950s.

Welsh star Rhys will serve as a producer on the project as well as starring.

Matthew Rhys and Keri Russell in The Americans

The actor recently finished up on final season of the critically-acclaimed FX spy drama The Americans, in which he starred alongside Keri Russell.