Dakota Johnson and Charlie Hunnam have been cast in the lead roles in the 'Fifty Shades of Grey' film.

The little-known 23-year-old actress will portray Anastasia Steele, while the 33-year-old actor will play Anastasia's lover Christian Grey in Sam Taylor-Johnson's adaptation of the erotic novel.

'Fifty Shades of Grey' author E.L. James made the announcement on her Twitter account today (02.09.13).

Her series of tweets read: ''Good moaning all. Hold on to your hats ... I am delighted to let you know that the lovely Dakota Johnson has agreed to be our Anastasia in the film adaptation of Fifty ... So... Christian... The gorgeous and talented Charlie Hunnam will be Christian Grey in the film adaptation of Fifty Shades of Grey. (sic)''

Dakota previously appeared in 'The Social Network' - in which she shared a love scene with Justin Timberlake - and had small roles in '21 Jump Street', 'The Five-Year Engagement' and 'Beastly'.

She is the daughter of Don Johnson, best known for TV shows 'Miami Vice' and 'Nash Bridges', and his actress ex-wife Melanie Griffith.

It's thought producers cast Dakota because they wanted a young, up-and-coming actress to play the role of virginal Anastasia, who embarks on an intense voyage of sexual discovery with her S&M-loving boss Christian.

British actor Charlie's previous work includes a lead role in TV series 'Sons of Anarchy' and a starring part in Guillermo del Toro's blockbuster 'Pacific Rim'.

'Fifty Shades of Grey' will be produced by Universal for release in August 2014.