Prince Harry claims he was "stopped cold" after Prince William used a "secret code" they had for "times of extreme crisis" in a row on the day of Prince Philip's funeral.
An extract from his memoir Spare, which was featured during his tell-all interview with Tom Bradby on ITV this evening, goes into detail about a confrontation between the two brothers on the day they said their final goodbyes to their late grandfather.
The bust-up, which Tom describes as an "extraordinary and heartbreaking scene" in the book, sees Harry recall how William forced him to look into his eyes so he could tell him that he loved him and wanted him to be happy.
Harry then says he told his brother that he loved him too but that his "stubbornness is extraordinary" and pulled away.
He goes on to claim that William pulled him back and said: "Harold you must listen to me, I just want you to be happy, Harold, I swear, I swear on mummy’s life."
Harry said this stopped them both as William had "used the secret code, the universal password, ever since we were boys those three words were to be used only in times of extreme crisis, ‘on mummy’s life’.
He goes on to detail how they had reserved that vow for times when they believed one of them needed to be heard.
And Harry adds: "It stopped me cold, as it was meant to. Not because he'd used it, but because it didn’t work. I simply didn’t believe him."
Harry's interview on ITV tonight is the first of four primetime interviews about his controversial memoir ahead of the book’s launch.
The duke has also spoken to Anderson Cooper for 60 Minutes on CBS News on Sunday night, Michael Strahan of Good Morning America on Monday and Stephen Colbert on the Late Show on CBS on Wednesday morning UK time.
In the interview with Bradby, Harry also speaks about being unable to show any emotion when meeting mourners following the death of his mother in 1997.
He also admits to feeling “some guilt” when walking among the crowds gathered outside Kensington Palace, saying the only time he cried was at his mother’s burial.
A string of revelations has already been leaked from the memoir, Spare, which is due to be published on Tuesday.
Harry has come under fire for some of the claims in the book, including that William physically attacked him and called his wife, the Duchess of Sussex, “difficult” and “abrasive”, as reported by the Guardian.
The Sun has reported that as well as the first alleged physical attack by his brother in 2019, Harry also claims that a “steaming” and “shouting” William grabbed his shirt as the pair held peace talks with their father in the gardens of Frogmore Cottage in 2021.
Harry: The Interview is available to view on ITVX. Spare published by Penguin Random House will be available to buy in the UK on January 10.