The erotic novel was written by by mother of two Erika Mitchell using the pen name EL James (The Writer's Coffee Shop)
Bestselling erotic novel written by mother-of-two Erika Mitchell under pseudonym  EL James (The Writer's Coffee Shop)

The erotic novel Fifty Shades of Grey has become the fastest selling paperback since records began and is on course to sell one million copies by the end of 2012.

The book by first-time novelist EL James, which has been dubbed "mummy porn", became the quickest selling paperback to reach £550,000 sales and only the fourth paperback novel to sell 100,000 copies in one week.

The novel - the first instalment of a trilogy - has now beaten JK Rowling's Harry Potter series and Dan Brown's Da Vinci Code to the UK record.

It is also on course to spend its fourth consecutive week at the top of the official UK top 50 book chart.

The book focuses on the relationship between recent college graduate Anastasia Steele and billionaire businessman Christian Grey.

While it contains a number of graphic sex scenes, including some involving sadomasochism, it is basically a conventional love story.

It was written by mother-of-two Erika Mitchell, 49, a former television executive from west London, using the pen name EL James and originally started off as a piece of Twilight fan fiction posted on the interent, before going on to sell more than 15 million copies worldwide.

Its explicit sexual content led to it being banned by some libraries in the United States, which further fuelled publicity for the novel following online petitions and an outcry from free speech groups.

A spokesman for Nielsen Book-Scan, which compiles the official chart figures, said: "The success has not been driven by people who regularly buy books, but by people who buy maybe three or four books a year.

"The books have crossed into the mass market. Clearly, people aren't embarrassed by it anymore."

Mitchell, who has been married to scriptwriter Niall Leonard for 25 years, is reported to be worth almost £4m following book deals and a Hollywood movie contract.

On the phenomenal success that she has experienced, Mitchell said she has received letters from fans thanking her for spicing up their sex lives.