'Fifty Shades of Grey', an erotic novel about a relationship between a young female virgin and her rich older lover, has sold over 100,000 copies in a week alone in Britain, beating the likes of both the Harry Potter and the Twilight series.

The three books in the series by 49-year-old author and mother-of-two EL James -- whose real name is Erika Mitchell -- have held the top three spots on the British paperback book chart for the past six weeks, The Sun reported.

At first, most sales of the series were of the more discreet ebook version.

But as the trilogy's popularity soared, 'Fifty Shades of Grey' has been Britain's best-selling book for the past nine weeks.

The success of the books has been due to "frustrated middle-aged mums", the daily said.

Erika Mitchell is now reportedly four million pounds ($6.3 million) after signing book deals and a Hollywood movie contract.