Harry Potter star Emma Watson has replaced Scarlett Johansson in period drama Napoleon after the movie bosses wanted someone younger for the part.

The part of Betsy Balcombe, a young noblewoman who falls in love with exiled French military leader Napoleon on the isolated British isle of St Helena, was originally written for an older actress than Watson.

Johansson quickly signed up for the role. However, during the rewrites the part has been been written as younger and Johansson has stepped aside for Watson.

There is no bad blood between the Lost In Translation star and the film bosses however - she is still attached to the movie as a producer.

It will be Watson's first major film role following her success in the Potter movies. She will film Napoleon later this year and will follow it up with the last instalment of the JK Rowling-penned franchise, Harry Potter and the Deathly Hallows.