The novel, the first in an X-rated trilogy, has beaten JK Rowling and Da Vinci Code author Dan Brown to the UK record.
Initially, the story was sold as an e-book, but as word of mouth spead, physical book sales have escalated
Last week alone, the first instalment sold more than 100,000 paperback copies.
Fifty Shades of Grey has been Britain’s best-selling book for nine weeks, while the trilogy has held the top three spots on the UK paperback book chart for the past six weeks.
Philip Stone, who compiles book charts for The Bookseller magazine, said: “He said: ‘It is highly unusual for a book to sell so well without there being a film adaptation or the author being well known, like Dan Brown or JK Rowling.
“Clearly it has become acceptable for something like this to be talked about openly.”
