Following reports that Sienna Miller had joined the cast of Guy Ritchie?s next film, 'Sherlock Holmes', it is now being reported that she?s changed her mind because ex-fiance Jude Law is in negotiations to take on the role of Dr John H. Watson.

At the beginning of this month there was talk of Miller playing Irene Adler ? the fictional character of a headstrong woman who only appears in one of the stories by Sir Arthur Conan Doyle, called 'A Scandal in Bohemia', with mention of her in a few others ? but she has apparently decided to not involve herself with the film anymore, fearing that she would end up emotionally scarred by spending time in the presence of her former beau.

Miller and Law met on the set of 'Alfie' and were engaged on Christmas day, 2004. However, only months after making this commitment, Daisy Wright ? the nanny who was looking after Law's children from his earlier marriage to Sadie Frost ? publicly revealed that she had an affair with Law while he was filming ?All The King?s Men? in Louisiana.

'Sherlock Holmes' is currently in pre-production and is set to be released in 2010.