EL James's erotic novel has sold over 5.3 million physical and ebooks combined, overtaking The Highway Code. It has topped the chart for 16 weeks in a row.
"My main ambition when I signed the deal with Random House was to see my books in the shops," James said.
"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."
Managing director of Cornerstone Publishing Susan Sandon added: "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."
She continued: "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!"
Sequels Fifty Shades Darker and Fifty Shades Freed have sold 3.6 million and 3.2 million respectively, bringing the total trilogy sales in the UK to over 12 million.
James will sign copies of her books at Waterstones Piccadilly in London on September 6.
All three books will be published in new hardback editions on September 13, priced at £12.99 each.
Foreign rights have been sold for the book in 44 territories, and sales in the US and Canada have surpassed 20 million.
The film rights to the books have been acquired by Universal Features and Focus Features.