The rift is said to be due to the pair's very different personalities. Photo / Getty Images
It was only a matter of time before all eyes would be on Kate and Meghan and how well they did – or didn't – get on. The royal sisterhood has not been under this much scrutiny since a young Princess Diana and an excitable Fergie joined the household in the Eighties.
As with those royal "out laws", the temptation to cast the modern-day Duchesses as arch rivals has proved too hard to resist. The obvious differences between their characters makes for an easy narrative: what, after all, would a Berkshire-born English Rose and homely mother of three really ever have had in common with a former American actress more at home on red carpets than in draughty royal residences?
As one insider put it: "They are very different women despite their similar circumstances. Meghan is an extrovert whereas Kate is quite shy."
Whether that is enough to make them mortal enemies is another matter - but it is certainly helping the rumours along that the Sussexes are moving to Windsor amid rumours of "tensions" with the Cambridges.
The contrast between them was plain to see when they appeared together at the first ever Royal Foundation forum in February. Meghan appeared to speak with more confidence than Kate, despite being the lower ranking newcomer. Asked if there were any disagreements with the family working closely together, William joked: "Oh, yes!"
Harry added: "Working as family does have its challenges, of course it does. But we're stuck together for the rest of our lives."
Rumours of disharmony between the so-called "Fab Four" had been rumbling behind palace gates before it was announced that Harry and Meghan would be moving to Frogmore cottage.
But even the Cambridges once described Kensington Palace as a 'goldfish bowl' and were able to escape to Anglesey and then Anmer Hall, their Norfolk bolthole, for the first five years of their marriage. So, as Harry and Meghan find their feet as a married couple and parents in waiting, it is perhaps understandable that they should seek to do so away from the royal "court". It is a special place for them too. They used Frogmore as a location for their engagement pictures upon the Queen's suggestion and its newly renovated grounds make an idyllic location to raise a family.
But Meghan's arrival onto the royal scene was always going to add to an already complicated family dynamic - and being an outsider was never going to make it easy.
While Kate grew up with the trials and tribulations of the family she married into, Meghan has always been a step removed from the soap opera playing out across the pond. As a woman from a completely different country, culture and background she has had to integrate herself into an alien world, where archaic procedures can sometimes be difficult to navigate. Kate had her mother's advice and nearly a decade to get to grips with palace protocol - Meghan has only had a year.
Which perhaps goes some way to explaining why her reported 5am wake up calls and email "bombardments" of staff may have raised eyebrows. What is typical in Hollywood, is not necessarily the norm in the "household", where there is an established hierarchy and a "way of doing things".
The sudden departure of Meghan's personal assistant, known only as Melissa, added to the speculation that the former Suits star is difficult to work for and "snaps" at staff. However her supporters insist the "charming" Duchess is "a breath of fresh air" and is just trying to get across her brief.
But the talk of a growing froideur between Kate and Meghan really ramped up following rumours of an apparent falling out between the pair in the run up to the Sussexes' wedding in May.
The Telegraph has spoken to two separate sources who claim Kate was left in tears following a bridesmaids dress fitting for Princess Charlotte.
"Kate had only just given birth to Prince Louis and was feeling quite emotional," said one insider.
The incident happened around the time Meghan was reported to have also "upset" the Queen by asking to wear an emerald tiara instead of the one offered by the 92-year-old monarch. It came after a book by veteran royal journalist Robert Jobson described Harry as "petulant and short-tempered" in the build up to the Windsor Castle wedding.
He allegedly told staff with a raised voice: "What Meghan wants, Meghan gets" and when reports reached the Queen, she asked to see her grandson privately and "put him firmly in his place", Mr Jobson wrote. Rumours of any lingering ill feeling between the Queen and Meghan appear wide of the mark, however. The two women, who share a love of dogs, enjoyed each other's company at their first solo engagement together in Cheshire in June - even if Meghan did inadvertently break protocol by not wearing a hat.
And earlier this month the Queen entrusted Meghan to "look after" Elke Budenbender, wife of the German president during the Remembrance commemorations in a move which saw her separated from the Duchess of Cambridge who was on an adjacent balcony with the Queen and the Duchess of Cornwall.
Despite being welcomed into the family fold, the attitude "below stairs" has apparently not been universally positive, with some giving the relationship "five years".
The catty gossip reportedly hit fever-pitch during the so-called "Markle Debacle" when Meghan's father Thomas pulled out of the wedding at the last minute. The drama reduced both Meghan and Harry to tears, which perhaps explains why the prince went to such great lengths to ensure his bride-to-be could have everything she wanted on her big day.
A source said: "Some of the staff don't seem to want to give Meghan a chance. Harry is besotted and understandably extremely protective of her."
Princess Eugenie's wedding also proved a little problematic for Meghan, who appeared to admonish her husband during the ceremony at St George's Chapel. The couple were later criticised for sharing news of their pregnancy at the champagne reception afterwards. William and Kate - not usually prone to public displays of affection - appeared much more at ease as they held hands throughout the service.
A source close to the Yorks told the Telegraph that Eugenie was "upset" that the Sussexes did not attend the evening reception, although Harry did pop along for a drink without Meghan. The Cambridges, meanwhile, "made a night of it", according to fellow guests.
However, one person who is said to be "completely taken" with Meghan is Prince Charles. Insiders say it is his new daughter-in-law who has encouraged Harry to have a closer relationship with his father and it is telling that it was Harry, not William, who delivered a heartfelt speech to mark his father's milestone at a special garden party in the summer. William and Kate were not present, choosing to visit Centrepoint - one of Diana's charities - on the day of Charles's actual birthday. Although both couples attended his birthday party that evening, they all reportedly left early.