Erotic fiction novel Fifty Shades of Grey has surpassed Harry Potter and The Da Vinci Code to become the fastest selling paperback since records began.
The novel, the first in an explicit trilogy by E L James, has become a word-of-mouth publishing phenomenon on its way to the UK sales record.
The book was the fastest paperback to reach 550,000 sales and is set to hit the million mark by the end of 2012.
Charlotte Bush, director of publicity for Arrow, which publishes the books in the UK, said: "The sales are incredible."
Last week alone the first book in the series sold more than 100,000 copies - a feat most of the Harry Potter books and all of the Twilight series failed to achieve - and it has been sitting at the top of the UK book charts for nine weeks.
The trilogy, which also includes Fifty Shades Darker and Fifty Shades Freed, started life as internet fan fiction based on the Twilight fantasy novels.
Written under a pen name by English mother-of-two Erika Mitchell, 49, the buzz surrounding the ebook version has seen physical sales reach 10 million copies worldwide.
The books - described by the author as "romantic fantasy" and as giving women "a holiday from their husbands" - tell the story of an affair between university student Anastasia Steele and her billionaire lover Christian Grey.
Erica Wagner, literary editor of The Times, told Sky News: "I think women are willing to take a punt on something exciting and it's grown from there."
A spokesman for Nielsen BookScan, which compiles the UK chart, said: "The success has not been driven by people who regularly buy books, but by people who buy maybe three or four books a year.
"The books have crossed into the mass market. Clearly, people aren't embarrassed by it any more."
However, in the US some libraries have decided to ban the book sparking an outcry from free speech groups such as the American Civil Liberties Union.
Two years after first publishing the books, Ms Mitchell is said to be worth £4m thanks to book deals on both sides of the Atlantic and a lucrative Hollywood film contract.
