Only four months after its publication in book form experts say Fifty Shades of Grey is outselling Dan Brown's The Da Vinci Code and JK Rowling's Harry Potter books, the Daily Mail reported.
EL James's erotic novel, released as an e-book last June and in book form in April, has sold 5.3 million print and digital copies, according to publishers Random House.
The other books in the trilogy, 'Fifty Shades Darker' and 'Fifty Shades Freed', have sold 3.6 million and 3.2 million copies.
Industry experts Nielsen Bookscan began compiling figures in 1998, and since 2003 'The Da Vinci Code' has held the record for the UK's best-selling book.
It has sold 5.2 million copies to date in the UK and the final Harry Potter instalment, 'The Deathly Hallows', has sold 4.6 million, said The Bookseller.
"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," EL James was quoted by the paper as saying.
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It is understood if global sales are taken into account JK Rowling's Harry Potter series is still more popular.
EL James' 50 Shades trilogy follows the relationship between a young student and a manipulative billionaire and features scenes of sado-masochism and domination.
It comes after the erotic novel outsold all seven Harry Potter books on Amazon last week, making author James the website's best-selling writer ever.
It also became the website's biggest-selling book of all time, eclipsing the sales of the previous record holder, 'Harry Potter and the Deathly Hallows', by more than two books to one.
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.