Sister reached the astonishing milestone earlier this month and led to its author Rosamund Lupton picking up the Nielsen Gold Award.

Since its publication in September 2010, Sister spent 14 weeks in the top 10 of the Sunday Times Bestseller list and broke sales records at WH Smith as the fastest selling title by a debut author in the retailer’s history. It was also crowned winner of the Richard and Judy Best Debut Novel of 2011 Award.

The novel – which explores the unbreakable bond between two siblings – was also publisher Little, Brown’s bestselling fiction e-book title of last year, with sales exceeding 55,000 copies. It has been translated into 31 languages and the film rights have been sold.

“I am really chuffed and a bit overwhelmed,” said Rosamund, who grew up in Little Chesterford and still regularly visits the area to visit her family.

“It has been a thrilling ride which goes on and on. I am terribly grateful and I’m amazed that so many people bought my book.”

Rosamund’s follow up novel, Afterwards, published in 2011, is also doing very well, particularly for a second novel, and has sold over 150,000 copies, although the mum-of-two says she is now looking to “take a breather”.

“It has been a whirlwind couple of years and very exciting,” she said. “Afterwards was written in nine months which was quite hard. I’ll be taking some thinking time.”

Nielsen is the worlds’ largest continuous sales analysis service and its book awards, established in 2000, are presented to authors and publishers that have achieved exceptional sales.