Harry describes 'heartbreaking' moment William used 'secret, three-word code' about Diana

Prince Harry has described the "heartbreaking" moment he claims his brother used a "secret code" referring to their mother that they had reserved for "times of extreme crisis".

During his interview with ITV ahead of the publication of his memoir Spare on Tuesday, Harry was asked by Tom Bradby about the description of his relationship with his brother, culminating in what the broadcaster dubbed an "extraordinary and heart-breaking scene" on their day of their grandfather's funeral.

An except from the book describes how Prince William had sworn "on mummy's life" that he wanted his younger sibling to be happy – using a "universal password" that the brothers had reserved for "times when one of us needed to be heard".

In the excerpt, Harry writes: "I pulled away, refused to meet his gaze. He forced me to look into his eyes, 'Listen to me Harold, listen, I love you Harold, I want you to be happy', the words flew out of my mouth, 'I love you too, but your stubbornness is extraordinary’, 'And yours isn’t?' I pulled away again.

"He grabbed me again, twisting me to maintain eye contact. Harold you must listen to me, I just want you to be happy, Harold, I swear, I swear on mummy’s life’.

"He stopped. I stopped. Pa stopped.

Watch: Harry admits he still questions Diana’s ‘unexplained’ death

Read more: Prince Harry reveals joke he shared with William as they walked behind Queen's coffin

"He’d gone there, he’d 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'.

"For nearly 25 years we’d reserved that soul-crushing vow for times when one of us needed to be heard, to be believed quickly, for times when nothing else would do.

"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."

Much of the interview is spent discussing his sibling rivalry with William and their fractured relationship.

He says he had always hoped he would form a tight-knit group of four once he had a long-term partner, but that William and Kate did not warm to Meghan from the start and that "stereotyping" caused a “bit of a barrier” them welcoming his future wife.

He told Bradby: “The fact that I had that in the back of my mind, and some of the things that my brother and sister-in-law, some of the way that they were acting or behaving definitely felt to me as though unfortunately that stereotyping was causing a bit of a barrier to them really sort of introducing or welcoming her in.”

When asked what exactly he meant by stereotyping, Harry elaborated: “American actress, divorced, biracial.”

Harry also discussed his mother frequently, saying he struggled to remember anything before her death while writing the book.

He said: "I lost a lot of memories, on the other side of this mental wall. Um, which again, I think is so relatable for so many people who’ve experienced loss, especially as a youngster, um, that inability to be able to like drag the memories back over. Um, but I think a lot of it was a defence mechanism."