Harry's book has garnered attention around the world with its disclosures about his personal struggles and its accusations about other royals, including his father King Charles, stepmother Camilla and elder brother Prince William.

"We always knew this book would fly but it is exceeding even our most bullish expectations," Transworld Penguin Random House Managing Director Larry Finlay said in a statement.

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

Citing British sales figures, the publisher said it had sold 400,000 copies so far across hardback, e-book and audio formats.

Earlier in the day, Caroline Lennon, a retail worker and one of the eager readers who had headed to bookshops to get their copy on the first day of its release, said she would read the book immediately as she posed for photographers.

"I like him, I like the royal family," said Lennon, 59, the first and only person waiting to buy a copy from a Waterstone's bookshop in central London when it opened.

Despite the lack of queues, Waterstone's said there had been strong pre-orders for the memoir which currently ranks as the best-seller on Amazon's UK, U.S., Australian, German and Canadian websites.

"I know perhaps some of the things he says have rubbed different people the wrong way," Lai Jiang told Reuters after buying a copy in Singapore.

"And I know, definitely, there are a lot of people who say that he shouldn't come out and say the things he says, but I believe Harry should be given a chance to say what he wants to say."

"Spare" is the latest revelatory offering from Harry and his wife Meghan since they stepped down from royal duties in 2020 and moved to California to forge a new life, and follows their Netflix documentary last month.

The royal family has not commented on the book or the interviews and is unlikely to do so.

Extracts from the book were leaked last Thursday when its Spanish language edition also went on sale by mistake in some bookshops in Spain.

Harry speaks of his grief and growing up after the death of his mother Princess Diana when he was just 12, his use of cocaine and other drugs to cope, how he killed 25 Taliban fighters while serving as a soldier in Afghanistan, and even how he lost his virginity.

He also reveals a heated row with William, the heir to the throne, saying his brother knocked him over, and how they had both begged his father not to marry Camilla, who he wed in 2005 and is now the queen consort.

In TV interviews ahead of the book launch, Harry has doubled down on his accusations that some royals, including Camilla and William, leaked stories to tabloid papers which had damaged either him or his wife Meghan in order to protect themselves or enhance their reputations.

"I think she (his mother Diana) would be heartbroken about the fact that William, his office, were part of these stories," he told Good Morning America (GMA).

In another interview with CBS show 60 minutes, he said Camilla had been a tabloid "villain" and needed to rehabilitate her image, which made her "dangerous".

"I don't regard her as an evil stepmother. I see someone who married into this institution and has done everything that she can to, you know, improve her reputation and her own image," he told GMA.

While Harry's revelations have dominated the headlines in the British media over the last week, the interest in his disclosures is far from universal.

"I was not planning to (read the book) as it so happens, or certainly not as an early priority," business minister Grant Shapps told Times Radio on Tuesday. "I've got one or two other things to do."