The saucy book, written by author E. L. James, recently became the fastest selling paperback of all time and earlier this year (12) it was announced bosses at Universal and Focus Features were planning to bring the tome to the big screen.
Now British star Marcel, who penned Tom Hanks' upcoming project about the production of Disney classic Mary Poppins, Saving Mr. Banks, has been tapped to write the script.
The Social Network producer Michael De Luca, who will also work on the film, tells industry publication Variety, "Kelly's work demonstrates her flawless structural technique and passionate commitment to emotion, humour and depth of character, which is particularly visible in the celebrated screenplay for the upcoming Saving Mr. Banks."
Casting for the romantic thriller, which follows the exploits of kinky lovers Anastasia Steele and billionaire businessman Christian Grey, has not yet been revealed, but fans of the book favour the likes of Harry Potter star Emma Watson and The Vampire Diaries hunk Ian Somerhalder for the leads.