The Duke of Sussex’s memoir is set to become one of the biggest books of the year, according to an industry expert, as it went on sale at half-price.

Philip Jones, editor of The Bookseller, told the PA news agency it is “quite normal” to have a “big hardback” that booksellers think will do well sold at half-price by major retailers.

Harry’s controversial autobiography Spare is being sold for £14, not the recommended retail price of £28, in bookshops such as Waterstones and WH Smith as well as online at Amazon.

His publisher Penguin Random House’s imprint Transworld said on Tuesday so far 400,000 copies have been sold across hardback, ebook and audio formats on its first day of publication, making it the fastest selling non-fiction book ever.

Larry Finlay, managing director of Transworld Penguin Random House, said: “We always knew this book would fly but it is exceeding even our most bullish expectations.

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

Spare was first leaked in Spain, where it mistakenly went on sale last week, and a host of revelations and private details about Harry’s life and family have since emerged.

Allegations from the book include his brother, the Prince of Wales, physically attacking him, and Meghan upsetting the Princess of Wales by saying she must have “baby brain” in the run-up to the Sussexes’ 2018 wedding.

Speaking before the sales announcement, Mr Jones said he believed Waterstones when the retailer said Spare had been one of its biggest pre-order titles of the past decade.

He added: “It is likely to be one of the biggest books of the year in terms of sales and impact.”

Asked how many copies a book has to sell to do well, Mr Jones said if it reaches 50,000 it will be “number one” and a “success”.

On Tuesday, Spare was number one in the UK Amazon chart and was listed as a bestseller on the company’s audiobook arm Audible and on Waterstones’ website.

For independent bookshops, low sales of the memoir would still be thought of as quite good, he added.

Mr Jones said more than 200,000 sales would make it one of the biggest books of the year.

Spare has surpassed this estimate by doing twice the number of sales before its first day on the shelves in bookstores ends on Tuesday.