Prince Harry has said learning of the death of his best friend in a car crash took him back to when he found out his mother had been killed.
The Duke of Sussex's controversial memoir, Spare, officially went on sale on Tuesday, and it contains numerous references to his friend Henry van Straubenzee, nicknamed "Henners", who died aged 18 in a car crash in 2002.
In his book, Harry said hearing the news of his friend's death took him back to the moment in 1997 when his father, then Prince Charles, told him Diana, Princess of Wales had died.
Harry and Henry had been close friends at Ludgrove School in Middlesex. Henry was a passenger in a car that struck a tree after leaving a party near Ludgrove on 14 December 2002.
Watch: Harry's book Spare goes on sale in London
In Spare, Harry describes how he found out his friend had been killed, comparing it to learning of his mother's death in a car crash in Paris in August 1997.
Harry wrote: "The news must have come in the form of a phone call, though I only dimly recall holding the phone, hearing the words.
"Henners and another boy, leaving a party near Ludgrove, drove into a tree. Though the call’s a blur, I vividly remember my reaction. Same as when Pa told me about Mummy.
"Right... so Henners was in an accident. But he’s in hospital, right? He’s going to be OK? No, he wasn’t. And the other boy, the driver, had been critically injured.
"Willy [Prince William] and I went to the funeral. A little parish church down the road from where Henners grew up. I remember hundreds of people squeezing into creaky wooden pews. I remember, after the service, queueing up to hug Henners’s parents, Alex and Claire, and his brothers, Thomas and Charlie."
Harry details the crash, saying his friend hadn't been wearing a seatbelt, adding: "Just like Mummy."
Despite their current rift, detailed in Spare, Harry and William, now the Prince of Wales, have been patrons of the Henry van Straubenzee Memorial Fund since 2009.
The fund invests in schools in Uganda, where it has completed more than 1,700 projects since 2003, raising about £3m.
Henry had just completed his A Levels at Harrow School before he died and had planned to take a gap year around East Africa.
He had a place at Newcastle University to study business management, and had won an army scholarship to the Royal Military Academy at Sandhurst, where he was due to attend after university.
He had just finished working as a junior master at his prep school, Ludgrove, when he died.
Harry makes a number of references to the pair's friendship in his memoir.
He writes of their school days together: "If I wasn’t roaming the corridors, I was roaming the school grounds, usually with my best mate, Henners. Like me, Henners was officially a Henry, but I always called him Henners, and he called me Haz."
In one particularly poignant moment, Harry outlines going to a 'pick-your-own' farm, adding: "Whenever I bite into a strawberry I’m there again, in those furrows, with lovely Henners."
Harry writes about how he missed his best friend when the prince went to Eton and Henry went to Harrow.
"Over time I’d scratched and clawed my way to the top of the Ludgrove pyramid," writes Harry. "Now, at Eton, I was at the bottom again. Starting over. Worse, without my best friend, Henners. He was attending a different school."
Harry also writes about how he visited his friend in Edinburgh in autumn 2002, just two months before he died.
The pair talked about their upcoming gap years and planned to meet in Africa.
Harry writes: "Maybe, we said, we’ll find ourselves side by side one day, marching into battle or helping people on the other side of the world. The future. We wondered aloud what it held. I worried about it, but not Henners. He didn’t take the future seriously, didn’t take anything seriously. Life as it comes, Haz. That was Henners, always and forever. I envied his tranquillity."
Harry would become good friends with Henry's older brother, businessman Thomas van Straubenzee, who is godfather to William's daughter, Princess Charlotte.
In Spare, Harry recalls a phone conversation with Thomas about his younger brother.
Harry wrote: "He was pleased to have someone to chat with while he was making his way home. His voice, so much like his brother’s, was an instant comfort.
"The conversation went inexorably to that original challenge, the wellspring of all challenges, Henners. Thomas missed his brother so much. Me too, I said. Man, me too. He thanked me for speaking at an event to raise money for Henners’s charity.
"Then we reminisced, randomly. Thomas and Henners, Willy and me, Saturday mornings, lounging around with Mummy, watching telly – having burping contests. She was like a teenage boy! She was, mate.
"Me and Henners mooning the security cameras at Ludgrove. We both started laughing. He reminded me that Henners and I were so close, people called us Jack and Russell. Maybe that was because Willy and I had Jack Russells? Oh I wondered where Henners might be. Was he with Mummy? Was he with the dead from Afghanistan?"
Watch: Harry admits he still questions Diana's 'unexplained' death