The fourth instalment in the erotic franchise, named Grey, has become the UK's fastest selling adult book after shifting more than one million copies in a week.
It gives protagonist Christian Grey's point of view on the events of the first book, which was originally told from Anastasia Steele's perspective.
Author EL James said she wrote Grey after being inundated with fan requests to delve deeper into the mind of brooding, mysterious billionaire businessman Christian, the man with a penchant for BDSM and control.
The author's publisher Susan Sandon said she was "delighted" with the early sales results, while her US publisher Anne Messitte said Christian's story was clearly proving "irresistible" to fans.
Grey has sold around 1.1 million copies in print, ebook and audiobook in the week since it went on sale. Publishers are now expecting to make second, third, fourth and fifth printings of the novel to meet demands.
Fifty Shades of Grey and its sequels Fifty Shades Darker and Fifty Shades Freed follow Christian's attempts to introduce Ana to the world of BDSM and make her into his willing submissive.
Watch a scene from the Fifty Shades movie below: