Top Royal author Tom Quinn claims Meghan allegedly left Kate "horrified" after a rant at Palace staff.
He told the Daily Star the 'foot-stamping' complaint at a member of the household appalled William's wife.
Quinn claims to has spoken to insiders and believes it was the start of a "rift" between the couples.
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And he claims it even led to them trying to avoid each other because it had led their relationship to become "frosty".
The author, who penned Kensington Palace: An Intimate Memoir from Queen Mary to Meghan Markle, has told all about the moment things became tense.
"After a couple of incidents, the foot-stamping incident and getting cross with one of the people that worked for Kate, there was a kind of frostiness because, as I understand it from my contacts, Harry felt that Meghan was being judged harshly.
"(Harry thought) She's new to the royal family, you've got to make allowances for the fact that she is going to get annoyed sometimes, that she's not going to understand, she can maybe have tantrums.
"Inevitably Harry backed Meghan and that caused a bit of a rift. Some of this is well-know with William, so when you've got the two couples who are slightly thinking the other couple is difficult, you've got a kind of frostiness that developed and they tried to avoid each other.
"They had all been part of a charity that was set up by William and Kate but Harry and Meghan parted too, and they split.
"That was such an obvious sign that things were not going well. So really it's just been those initial incidents so they ignored each other because they didn't want it to happen anymore."
Rumours of a rift were heightened when Meghan and Harry sensationally stepped down as senior royals at the start of the year.
They moved to Los Angeles via Canada with baby Archie, and due to the coronavirus pandemic they have not returned to the UK since.
They split from a charity they shared with William and Kate, the Royal Foundation, and have since started their own organisation named Archewell.
But according to Mr Quinn, Meghan and Kate fully intended to become friends but found the huge expectations on them to be a challenge.
Mr Quinn continued: "Initially I think both Kate and Meghan thought they might become really good friends. Then there was this foot-stamping incident, the shouting at the servant.
"Then the frostiness. It is a great shame. The sad thing isn't that they didn't become friends but, in a way the greater shame is that they were under pressure to become good friends, and why should they become good friends?
"They are only thrown together by Kate marrying William and Meghan marrying Harry. I think they felt too there was a lot of pressure for them so they were terrified.
"Apparently when they went to Wimbledon and sat next to each other, during the early period before they had fallen out, and there were jokes about 'we must be really careful that we smile at each other all the time or the press will think we've fallen out'.
"If you look at the shots, they do make the effort to smile and leaning closer to each other.
"But that was in the early period when they put on a good show, but even in that early period these were two very different people and it's difficult if two people don't have anything in common other than the fact they are both members of the firm, it's very difficult for them to get on."
Both couples have been approached for a response.