Fifty Shades is UK's best-selling book of all time

Author E L James is looking to adapt her best-selling erotic novel, Fifty Shades of Grey, for the big screen (AP)

E L James' Fifty Shades of Grey has sold more than 5.3m copies making it the best-selling British book of all time, according to figures released by its publisher.

The book, the first of a trilogy of steamy tales dubbed "mummy porn" has become a literary phenomenon.

Ms James said said: "My main ambition when I signed the deal with Random House was to see my books in the shops. I simply had no idea they would be so successful and this is totally unexpected.

"The whole process has been both extraordinary and rewarding. I couldn't be more pleased with the sales and would like to thank everyone involved."

The second and third books in the trilogy, Fifty Shades Darker and Fifty Shades Freed, have sold around 3.6m and 3.2m copies respectively.

Susan Sandon, managing director of Cornerstone Publishing which is part of Random House, said: "The Fifty Shades of Grey phenomenon is perhaps one of the most extraordinary experiences of my entire publishing career and I feel privileged to be part of it.

"The speed and sheer size of the sales is incredible and the roles played by so many people across the business simply breathtaking.

"Each and every department across editorial, art, finance, legal, publicity, marketing, production and home and international sales has risen to the creative and logistical challenges presented by the trilogy with energy, excitement and passion.

"I take my hat off to the brilliance and professionalism of all involved. And above all, it is so much fun."

Rights to the book have been sold around the world and the trilogy is being published in languages including Albanian, Chinese, Russian, Serbian and Vietnamese.

All three books are due to be filmed as part of a movie deal.

PA