The Princes are barely speaking during a crucial time in the history of the monarchy.
A “hurtful rift” which both parties have “refused to put to bed”, has led William’s wife Kate to try to repair “a complete and utter breakdown of communication” between the brothers.
Her advice led to Harry and Meghan inviting them to their new home at Windsor after they all attended church on Easter Sunday.
They had tea and chatted for around 30 minutes at Frogmore Cottage .
Kate forged a “very strong friendship” with Harry after she and Wills worked closely with him to launch the Royal Foundation and the mental health charity Heads Together.
They were neighbours at Kensington Palace before Harry married Meghan, with William once joking that he would often come over to raid food from the fridge.
A palace source said: “The Duchess and Harry have developed a very close bond over the years.
“Kate is acutely aware of the importance of William and Harry getting along, not just because of perception but their collective responsibility to the monarchy as a whole.
“Harry has acted as counsel to his brother throughout William and Kate’s more turbulent years, and the three of them had gone on to form the most tremendous partnership.”
Insiders said the brothers had a “disastrous row that could not be defused” over the different paths in life they are now on.
While William is being prepared to become King one day, Harry is forging his own path which “before meeting his wife Meghan had been severely lacking”, another source said.
Royal watchers noticed the brothers appeared to avoid each other at the Easter Sunday service, and stood either side of Zara and Mike Tindall before greeting the Queen at St George’s Chapel.
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In contrast, at the Anzac Day commemorations at Westminster Abbey today, Harry and Kate chatted and smiled, looking completely comfortable together.
William was in New Zealand, where he met five-year-old Christchurch terror attack survivor Alen Alsati and greeted New Zealand PM Jacinda Ardern with a traditional hongi nose rub.
The princes have long carried a heavy burden as “saviours” of the monarchy.
Harry has revealed that “no one wants to be King” but the family is committed to the throne “for the greater good of the people”.
The brothers created an image which became seared on the national consciousness as boys walking behind their mother Diana’s coffin at her funeral 20 years ago.
As they transformed in handsome, confident men, first in the armed forces and then with their charitable roles, their bond seemed unbreakable.
But Harry has spoken in the past about “making something” of his life himself.
He said: “I feel there is just a smallish window when people are interested in me before [William’s children] take over.”
Courtiers’ hopes the two couples would become a “Fab Four” highlight joint causes have also been dampened since Harry and Meghan’s move from Kensington Palace.
A plan for the Duke and Duchess of Sussex to carry out large charity and conservation projects in Africa, which could see them move abroad for up to six months at a time, has also added to the worrying situation that has developed.
One courtier said: “If Harry and Meghan decide to live in Africa for months at a time, the divisions could expand rapidly.
“The Sussexes are intent on forging their own path, apart from the constraints they feel restrict the Cambridges.
“This is not even much to do with the differences between them and their wives, a lot of which has been overplayed by people who don’t know much about the situation.
“Any issues between William and Meghan are wide of the mark but a specific conflict has led to a deep and personal falling out that could not be defused nor abated.
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It is said the Queen and the Prince of Wales have even plotting to stage an intervention.
Another source said: “There is a very real fear that if the princes do not repair their relationship, it may reach a point where it is too late.
“Not only would that be hugely damaging to them and their families, it would be potentially catastrophic as we move into the most revealing era for the royals.
“They are the future of the monarchy, its standing in Britain, throughout the Commonwealth and the world.
“After the years of negativity from the generation before them, behaviour that fuelled constant headaches and heartache, there was hope that the brothers would continue to be each other’s stay and support.
“But that seems to have completely faded now.
“It can be very lonely at the top and sometimes William is left feeling that he is facing such tremendous responsibility alone.
“His wife is a great support, but what they have experienced as children and the history they share is incomparable.
“At the very core, that is what has hurt them both so deeply.
“But at one point, one of them is going to have to swallow their pride and admit there is a problem that needs addressing, before all is lost.”