Harry Potter rehearsal script is UK's fastest-selling book this decade

The script for a West End play is outselling Fifty Shades of Grey's opening week....

It might be a script for a West End play that wasn't even penned by JK Rowling, but Harry Potter and the Cursed Child Parts I & II is proving to be quite the smash hit in British bookstores.

Having sold over 680,000 copies in its first three days on shelves, it is officially the fastest-selling book in the UK this decade.

It has easily outsold the massively popular Fifty Shades of Grey, which achieved first week sales of 664,478 in 2012.

The title for biggest single-week book sales since British records began is currently held by another Potter title. JK Rowling's final novel in the series, Harry Potter and the Deathly Hallow, sold 1.8 million copies – as well as 780,000 copies of the version intended for adults – in its first week back in July 2007.

...The Cursed Child looks set to become the second biggest-seller ever by the end of this week.

Written by Jack Thorne from a story by Thorne, John Tiffany and Rowling herself, it is already the fastest-selling script book of all time, according to publishers Little, Brown.

North America publisher Scholastic has confirmed that it has sold over two million copies across the Atlantic in its first two days on sale.

The book currently on sale is the "Special Rehearsal Edition", based on the script used in summer previews.

A "Definitive Collector's Edition" will arrive next year.

Meanwhile, the West End play opened in London's Palace Theatre on Saturday. Though all performances are currently sold-out, 250,000 new tickets are set to be released tomorrow.

While the play has received strong reviews, the print version hasn't won over all fans...