But it wasn't a feud that happened overnight. Bad blood has been brewing for years following well-documented jealousies and misunderstandings, which intensified with the arrival of Harry's wife Meghan. Things only got worse after the couple stepped down as working royals and hit out at their royal relatives in a docuseries on Netflix, followed by the duke's memoir Spare.
And now royal author Omid Scobie's new book Endgame looks set to drive even more distance between the two brothers, with sensational claims about William's alleged temper and story leaking featuring heavily. Here, The Mirror breaks down exactly what happened to turn the once inseparable brothers into enemies...
September 1998: 'William snubs Harry at Eton'
In his memoir Spare, Harry claims William was keen to create a distance between them as far back as secondary school. When he joined his big brother at elite boarding school, Eton College, Harry claimed William told him to pretend they didn't know one another, and alleged that his older brother "hated it when anyone made the mistake of thinking us a package deal". Harry described how he wanted to share things with his brother, who he says seemed "indifferent" to him.
January 2005: 'Nazi costume scandal'
According to Harry, it was William and Kate - who was the future king's new girlfriend at the time - who encouraged him to don the now-infamous uniform to a fancy dress party. Whilst it was, according to the duke, their encouragement that caused him to choose it, he faced the backlash alone - and it filled him with "bottomless self-loathing".
In the passage in his memoir he also described making peace with the idea that Kate may take his elder brother's time from him by imagining the future good times they would all share together.
November 2010: 'William's engagement is news to Harry'
In November 2010 it was announced that Kate and William were getting married, but this was news to Harry. In Spare, he rubbished claims he gave his older brother their late mother's engagement ring so he could use it to propose to Kate Middleton, labelling the story "absolute rubbish".
Following the now Prince and Princess of Wales' engagement announcement in November 2010, it was reported that Harry had shared a "tender moment" with his elder brother, prior to William's engagement. But Harry said that wasn't the case, and claimed he didn't even know his brother was planning to get down on one knee.
According to the prince, he found out about William's engagement via newspapers, despite spending a recent trip together in Lesotho. He wrote: "Soon after we returned to Britain the Palace announced that Willy was going to marry. November 2010... News to me. All that time together in Lesotho, he'd never mentioned it.
"The papers published florid stories about the moment I realised Willy and Kate were well matched, the moment I appreciated the depth of their love and thus decided to gift Willy the ring I'd inherited from Mummy, the legendary sapphire, a tender moment between brothers, a bonding moment for all three of us, and absolute rubbish: none of it ever happened. I never gave Willy that ring because it wasn’t mine to give. He already had it. He'd asked for it after Mummy died, and I'd been more than happy to let it go."
April 2011: 'William's best man riddle'
In 2011, Kate and William finally tied the knot, but Prince Harry claimed that despite being the public-facing best man, the honour of giving the speech at the reception was left with two of William's friends instead. In his memoir, Harry said he understood why the privacy of William's friends needed to be respected, but said that despite what the public thought it was a "bare-faced lie" that Harry held this position of honour.
Summer 2013: William 'jealous' of Harry's new project
As William was about to become a father, Harry took on a new project of his own - the Invictus Games. However, in his memoir, he accused his older brother of reacting with envy and the hallmarks of "sibling rivalry" when he told him about the new charitable endeavour. He also claimed that when he got involved in efforts to prevent illegal poaching in Africa in late 2015, his brother reacted by saying "Africa was his thing".
Summer 2016: Harry introduces Meghan
Although he had already introduced his new girlfriend Meghan to his cousin Eugenie, when he admitted to his brother and sister-in-law that he was in a relationship, they reacted with shock and joy and told Harry that they were massive fans of the TV show she stared in - Suits. Harry said he looked forward to the upcoming experiences he and Meghan would share with William and Kate, but his brother warned him to take things slow.
In October 2016, Meghan met William for the first time, Harry explains in his memoir, and quickly went in for a hug - something he says made William recoil. Harry put this down to a culture clash and said noted that his brother told him sincerely that he was happy for him and his new relationship, which was made public around the same time.
November 2016: 'Row over Harry's statement defending Meghan'
In November 2016, Harry released a strongly worded statement about the backlash Meghan was facing since their relationship was made public - something that he claimed in his memoir made both his father Charles and William extremely angry.
He wrote: "Pa and Willy were furious. They gave me an earful. My statement made them look bad, they both said, 'Why in hell? Because they'd never put out a statement for their girlfriends or wives when they were being harassed."
Autumn 2017 - 'William warns Harry not to marry Meghan'
In his memoir, Harry says his brother advised him against his plan to propose to Meghan, with his elder brother warning him not to move too fast.
After a year together, Harry was ready to get down on one knee which, according to Spare, did not go down well with his elder brother. He is said to have been 'concerned' and told his younger brother he was taking things 'a bit too far'. Harry wrote: “It's too fast, he'd told me. Too soon. In fact, he'd actually been pretty discouraging about my even dating Meg.
"One day, sitting together in his garden, he'd predicted a host of difficulties I could expect if I hooked up with an 'American actress', a phrase he always managed to make sound like a 'convicted felon'. According to author Omid Scobie, the wedding marked the beginning of a more distant chapter in the brothers' relationship.
June 2018: 'The Fab Four try to patch things up'
Harry said he and Meghan were invited to William and Kate home in an attempt to build bridges after the wedding. While there, Harry claims his brother lost his temper and pointed in Meghan's face after in response to her claiming Kate had "baby brain".
Scobie reports in his book Endgame that Charles stepped in during the festive season of 2018, to try and repair relations between the two brothers, and advised William to invite Meghan and Harry to Anmer Hall - his Norfolk home - for the Christmas period. William reportedly did so, but according to Scobie "the band-aid didn't hold for long".
Early 2019: Brothers come to blows
Prince Harry claimed his brother grabbed him by the collar and knocked him to the floor in a brawl following a heated row over Meghan. In Spare, the Duke of Sussex alleged he had a confrontation with his elder brother at his London home in 2019 which left him with cuts and scratches on his back.
The now Prince of Wales is said to have arrived at Harry's Nottingham Cottage home to talk about their relationship already "piping hot", with their pair shouting over each other in the squabble. Harry then dismissed his brother's claims he was trying to help and went to the kitchen where he allegedly told him "Willy, I can't speak to you when you’re like this".
He claimed: "He set down the water, called me another name, then came at me. It all happened so fast. So very fast. He grabbed me by the collar, ripping my necklace, and he knocked me to the floor. I landed on the dog’s bowl, which cracked under my back, the pieces cutting into me.
"I lay there for a moment, dazed, then got to my feet and told him to get out." The Prince of Wales reportedly asked his brother not to tell Meghan about the confrontation in the aftermath, resulting in Harry saying: "You mean that you attacked me?", to which he responded: "I didn't attack you, Harold."
Harry said he did not immediately tell his wife about the incident but she had later noticed 'scrapes and bruises' on his back. During the argument William is said to have called Megan "difficult", "rude" and "abrasive" – comments which the younger brother said parroted 'the press narrative' about his wife.
October 2019: 'Different paths'
Harry admitted during an interview with ITV that he and his brother are "on different paths at the moment" after there had been speculation in the press over the state of their relationship, although the duke said he loved his brother "dearly". In January 2020, the Sussexes announced their intention to step back as full-time working royals to create more of a hybrid mode.
Shortly afterward, William is quoted by the Sunday Times as saying "I've put my arm around my brother all our lives and I can't do that anymore; we're separate entities".
Later that month it is announced that there will be no hybrid model available to the Sussexes, and that they are fully out as working royals.
March 2021: Bombshell Oprah interview
The following year, Meghan and Harry gave their explosive interview to Oprah in which they detailed some of the reasons they had wanted to step back as working royals. In their Netflix documentary, which was released in 2022, Harry is seen receiving a text message from his brother and looking upset in a scene filmed right after the interview had aired.
In the final episode of the six-part docuseries, Meghan is heard cutting short a conversation with friend Tyler Perry, announcing: "Wow. H just got a text... H just got a text from his brother."
It launched Harry into a tailspin as he told his wife: "I wish I knew what to do," while Meghan responded: "I know. Let's take a breather. Get some air and then decide."
It came after Meghan and Harry accused an unnamed member of the royal family of raising 'concerns' about the colour of their son Archie's skin. William hit back, telling reporters at an engagement that we are "very much not a racist family." The Queen also releases the now-infamous "recollections may vary" statement.
April 2021: Brother's reunite for Philip's funeral
Given the pandemic, Harry and William's paths didn't cross for some time, until their grandfather, Prince Philip's funeral in April 2021. The socially distanced funeral saw only a handful of mourners attend, and after the service, William and Harry were seen tentatively chatting with Kate on the walk back to Windsor Castle.
Harry claimed in his memoir his brother called him out on his interview with Oprah, and his own cooperation with the press, before using a "secret code" that stopped Harry in his tracks. When either of the brothers said the words "on Mummy's life" it was a signal that they needed to be believed by the other, and in this case, William used to it reiterate to his younger brother that he wanted him to be happy.
July 2021: Diana's statue is unveiled by both brothers
A couple of months later the two brothers reunited again for another poignant occasion, the unveiling of a statue of their late mother on what would have been her 60th birthday. The statue, commissioned by the brothers, being unveiled was the last time they would be together for a long time.
All eyes were on Harry and Meghan when they returned to the UK for Queen Elizabeth's Platinum Jubilee celebrations. The Sunday Times reports that the brothers "spent no private time together" during the weekend.
September 2022: Queen Elizabeth dies
The brothers were reunited to mourn their grandmother Queen Elizabeth and viewed tributes left by the public together in Windsor, alongside their wives. However, Scobie reports that this was the first time the couples had seen each other since the Jubilee, and that the car ride there was uncomfortable to say the least.
According to the author, William was forced to invite Harry and Meghan on the walkabout by Charles. For Kate, the occasion is said to have been one of the "hardest things she'd ever had to do".
December 2022: Harry's Netflix documentary drops
As William was in Boston for the Earthshot prize, Netflix released the trailer for Harry and Meghan's Netflix docuseries. Scobie claims that the streaming giant timed this on purpose, and the series itself was filled with further details about the Sussexes' complaints and allegations against the royal households.
Scobie claims in his book that it was a frosty festive period, and he says that while the Sussexes sent Christmas presents to their nephews and niece, no messages were exchanged directly between the two brothers.
January 2023: Spare is released
Harry's memoir was released early this year and quickly topped bestseller lists. It detailed the difficulties the brother's had faced in their sibling relationship since childhood, and alleged that staff working for William had leaked stories about the Sussexes to the press. In interviews promoting the memoir, Harry said he was open to reconciliation if he received an apology, but the ball is in his family's court.
March 2023: Sussexes evicted
News is broken in March that the Susexes have been evicted from their home of Frogmore Cottage, although Scobie reports that this happened earlier and was a direct response to Spare's release. He claims Princess Anne was the one who convinced Charles to force them to hand back the keys. Harry also returns to the UK for some legal hearings, but doesn't see his brother or father. Two months later he attended King Charles's coronation, but Meghan stayed at home. William played a staring role in the ceremony and paid homage to his father, but Harry was relegated to enjoy the proceedings from three rows back.
Omid Scobie's latest book Endgame is packed with new insights and allegations from the author and long-time associate of Prince Harry and Meghan Markle. The hardback is reduced to £11 on Amazon but Mirror readers can listen to the audio version without paying a penny using a free Audible trial.