Nile Gardiner, a foreign policy analyst and former aide to Margaret Thatcher, predicted that the book would further damage the pair’s relationship.
Mr Gardiner wrote on Twitter: “Prince Harry’s memoir is set to be a nasty hatchet job attack on his own brother.”
Source familiar with the manuscript says Spare is ‘tough on William in particular and even Kate gets a bit of a broadside’
Spare has already spent weeks near the top of the Amazon books sales charts based on advance orders, with the title at No. 5 in the UK listings on Sunday.
The couple signed a $20 million, four-book deal with Penguin Random House in 2021 and there has recently been unsubstantiated speculation that a memoir by Meghan might form part of the publication series.
The publication of Spare is being accompanied by two major television interviews of the Prince by ITV’s Tom Bradby and CBS’s Anderson Cooper to be broadcast in the coming days.
Prince Harry is also expected to give at least one newspaper interview to accompany the launch, with The New York Times – in which Meghan wrote about her miscarriage in 2020 – a possible outlet.
But in an indication that the six-part Netflix series has backfired on the couple, a poll last week
found support draining away from them.
The YouGov poll found almost half of the public think the Duke should be stripped of his royal title, with 44 per cent saying they were more sympathetic towards Prince William and the Princess of Wales, compared with just 17 per cent for Prince Harry and Meghan, Duchess of Sussex.
Penguin Random House did not respond to requests for comment. ITV would not comment on the content of the Bradby interview.
A spokesman for the Sussexes did not respond to requests for comment on the content of Prince Harry’s book.