Dakota Johnson and Charlie Hunnam
Dakota Johnson and Charlie Hunnam to play Anastasia Steele and Christian Grey in the film adaptation of Fifty Shades of Grey

Fifty Shades of Grey fans have expressed disappointment at the announcement that Dakota Johnson and Charlie Hunnam have been cast as the leads for the erotic novel's film adaptation.

Author EL James, author of the bestselling bondage trilogy, revealed via Twitter that Social Network star Johnson had been given the highly coveted role of Anastasia Steele.

"I am delighted to let you know that the lovely Dakota Johnson has agreed to be our Anastasia in the film adaptation of Fifty Shades of Grey," she told her 353,000 followers.

Minutes later she revealed who would be playing the mysterious male protagonist, Christian Grey.

"The gorgeous and talented Charlie Hunnam will be Christian Grey in the film adaptation," she said.

But while James may have been excited about the casting, fans were lukewarm about the names.

"Not impressed. Both too old," one fan said.

Another wrote: "So not liking this. Henry Cavill is the only actor I had in mind for Christian Grey. I feel so disappointed now."

One unconvinced Twitter user posted Johnson's photo side-by-side with Alexis Bledel, one of the top fan favourites for the role.

Emma Watson, Shailene Woodley and Analeigh Tipton had also been mooted to the role.

Fifty Shades of Grey follows a young college student as she embarks on a sadomasochistic relationship with an older businessman, Christian Grey. Its success of made it the fastest selling book of 2012. It has sold more than 70 million copies worldwide.

The movie, which will be directed by Sam Taylor-Johnson, is set to hit cinemas in the US in August 2014.