Rowling's Harry Potter has been the strongest seller on Amazon since 1997, but now the Fifty Shades trilogy has outsold the entire seven-book Harry Potter series, making James the best-selling author of all time on Amazon.co.uk.
Erotic novel Fifty Shades of Grey is outselling Rowling's final Harry Potter book, Harry Potter and the Deathly Hallows, by two to one on the UK retail site.
In total, the Fifty Shades trilogy has surpassed 4m copies sold on print and Kindle, taking just four months to achieve the feat after sales started in March 2012.
Fifty Shades of Grey is also the best-selling book ever on the online retail giant's British web store.
"If JK Rowling was the literary phenomenon of the last decade then EL James looks certain to take that mantle in the current decade," said Gordon Willoughby, the EU director of Kindle.
"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."
James's Fifty Shades books also continue to top the best seller chart in print and on Kindle at both Amazon.co.uk and Amazon.com.
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