Russell Crowe is to star as Sherlock Holmes' sidekick Dr. Watson.

The 'American Gangster' actor will team up with Robert Downey Jr., who is to play the fictional British detective, in a new movie by Guy Ritchie.

Russell, 44, had agreed to play the Sheriff of Nottingham in 'Nottingham', a retelling of the story of Robin Hood - but after the project was put on hold he was able to commit to Guy's film.

A source told Britain's The Sun newspaper: "Russell wants the part, Guy wants Russell. All they've got left to do is dot the i's and cross the t's."

Guy has already promised his version of Holmes will be much "riskier" than previous portrayals and the film will "take advantage of Holmes' skills as a boxer and swordsman".

Robert recently revealed he wants the crime-solving duo to be a pair of "bad a**es".

The 43-year-old actor added: "I've got to spend some time with Guy and I love his take on it. We're both martial arts enthusiasts and in the original stories of Sherlock Holmes, he's kind of a bad-a*s and a bare-knuckle boxer and studies the rare fictional martial art of baritsu. It's a period piece where you don't modernise it, you just realise how modern it was."

The script for the film is based on a forthcoming comic book written by Lionel Wigram, a former Warner Brothers executive, who will also produce the movie.

Filming is expected to start next month, with the as-yet untitled film expected for a 2010 release.

Guy's latest film 'RocknRolla' is released in European cinemas from this Friday