Prince Harry's autobiography, "Spare," becomes the fastest-selling non-fiction book ever, according to Transworld Penguin Random House on Tuesday.
More than 400,000 copies across hardback, ebook, and audio formats on its first day of publication.
The managing director of Transworld Penguin Random House Larry Finlay 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)."
Long lines of people who wanted to purchase the book started before shops opened at 12 am Tuesday morning for the official release of the memoir.
Anticipating high customer demand, the booksellers opened their flagship branch in Piccadilly early on Tuesday, as the book has been published worldwide in 16 languages.
Branches of WH Smith in locations including Euston, Victoria, Heathrow, and Gatwick were among those extending their hours for the release.
The memoir is already atop UK bestseller for online retailer Amazon, days after headline-grabbing revelations emerged from leaks - from how Prince Harry lost his virginity to claims Prince Harry was attacked by his brother, Prince William.
The 410-page memoir, which reveals conflict and personal tensions within the royal palaces, shows Prince Harry's version of his upbringing and then falling out with the royal family.
So far Buckingham Palace and Kensington Palace have not responded.
However, allegations in the book include that Prince Harry begged his father not to remarry, that he killed 25 Taliban fighters while serving in Afghanistan, that he took drugs, partly in response to panic attacks, and that Meghan and Catherine had a difficult relationship.
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