Prince Harry's autobiography "Spare", is the fastest-selling non-fiction book ever.

On its first day of publication, the book sold 400,000 copies in hardback, ebook, and audio formats.

Larry Finlay, Transworld Penguin Random House's managing director, stated that "We knew this book would be a success, but it has exceeded even our most optimistic expectations."

"As far as we know, the only books to have sold more in their first day are those starring the other Harry (Potter)."

Harry's book was mistakenly released early in Spain last week with journalists scrambling to get a copy to report on what was inside before it was taken off the shelves.

It was officially released today, Jan 10, and immediately flew off bookshelves.

Before shops opened at 12 am Tuesday morning for the official release of the controversial memoir, queues were already formed in anticipation of the book.

A handful of people waited outside the doors of WHSmith in London's Victoria station to be one of the first to buy a copy of the book.

In the ghostwritten memoir Harry, 38, recounts his grief at the death of his mother, Princess Diana, his disputes with his brother Prince William and his frustration at the role of royal "spare" in the shadow of his elder brother, who is heir to the throne.