LONDON: An erotic book dubbed as “mummy porn” has beaten the J K Rowling authored Harry Potter ’s record in the UK book sales chart, becoming fastest selling paperback of all time in just one week.The book ‘Fifty Shades of Grey’ , about a depraved relationship between a young female virgin and her rich older lover has become the fastest selling paperback since records began.Erika Mitchell, who writes with the pen name ‘ EL James ’ beat JK Rowling and Da Vinci Code author Dan Brown in the UK records with the first instalment of her Xrated trilogy, the Daily Mail reported. It sold more than 100,000 paperback copies last week alone — a success story which most of the Harry Potter books and all the Twilight novels have failed to match.At first, most sales of ‘The Fifty Shades’ series were of the more discreet e-book version . Now, the book has been Britain’s best-seller for the past nine weeks.Experts say the success of the racy books, dubbed-“ mummy porn” has been driven by frustrated middleaged mothers. “The success has not been driven by people who regularly buy books, but by people who buy maybe three or four books a year,” spokesman for Nielsen Book-Scan , who complies the official chart figures, said.