EL James's Fifty Shades trilogy has outsold JK Rowling's seven-book Harry Potter series on Amazon's UK site.
James has sold more than four million copies of her books in print and on Kindle since they went on sale in March, making her the best-selling author of all time at Amazon.co.uk.
JK Rowling's final book in the Potter series, Harry Potter And The Deathly Hallows, is being outsold by the first book in the James trilogy, Fifty Shades Of Grey, by more than two to one on the website.
EU director of Kindle Gordon Willoughby said: "If JK Rowling was the literary phenomenon of the last decade, then EL James looks certain to take that mantle in the current decade.
"In just four months, EL James has become Amazon.co.uk's biggest-selling author of all time, which is truly remarkable when you consider that we've been selling books for almost 14 years."