They are the royal rumours which have left Kate boiling with rage and that threaten to ruin her relationships with close friends known as the "Turnip Toffs".
Rose is the Marchioness of Cholmondeley, whose husband David, the Marquess of Cholmondeley, is a close pal of William’s.
She is known as a “beguiling socialite” and a “glamorous ex-model” – and until this year the two couples got on well, living just 6km apart in Norfolk, near the Queen’s Sandringham estate.
The royals and the aristocrats are all part of a set of wealthy farmers and landowners known as the “Turnip Toffs”.
So what are the rumours which have torn them apart and hit the headlines in the British tabloids?
One friend close to both couples said: “The rumours started around Christmas that William had become just a bit too close to Rose and was infatuated with her.
“She is a naturally flirtatious and beautiful woman and William is only human.
“The rumours are untrue but they got back to Kate and she hit the roof."
“It’s a very small world in the aristocratic country set and social media amplifies everything.
“Pretty soon the rumours were swirling about on Facebook and WhatsApp devices and spread like wild fire.
“Even though they are not true, Kate clearly feels threatened.
“It’s very sad because they were close friends and Rose’s children played with Kate’s George and Charlotte."
“If the rift is as bad as it seems, it’s going to be hard living so close and moving in the same social circles.
“Lots of friends of both couples will find it very embarrassing and won’t know what to say to either of them.”
William and Kate moved to Anmer Hall in 2014 and soon became friends with Rose and David, who have twin boys, Alexander and Oliver, and a daughter, Iris.
Eton-educated former filmmaker David, who at 58 is 23 years older than Rose, inherited more than $200 million in 1990, including a castle in Cheshire and Houghton Hall, one of England’s finest Palladian houses, surrounded by 400 hectares of parkland close to Sandringham.
Now she sees him once a year as Lord Great Chamberlain at the State Opening of Parliament.
Rose’s paternal grandmother, Lady Elizabeth Lambert, was a bridesmaid at the Queen’s wedding in 1947, and Rose and her glamorous sister Marina were photographed with Tony Blair on a 2005 holiday in Italy.
Mutual friends are said to be “stunned” by the fall-out as Kate and Rose, both mums of three who both suffered from morning sickness during their pregnancy, had so much in common.
Both women are patrons of the charity East Anglia’s Children’s Hospices, and Kate has attended horse trials at Houghton Hall.
The Cholmondeleys hosted a dinner in 2016 and Kate was seen chatting away to her host. Rose sat next to Prince Harry at a Buckingham Palace banquet in 2017 and stole the show wearing an extravagant tiara.
One newspaper reported: “William wants to play peacemaker so that the couples can remain friends, given that they live so close and share many mutual friends."
“But Kate has been clear she doesn’t want to see them anymore and wants William to phase them out, despite their social status.”
Another newspaper claims the rumours of a falling out are false and were started just to damage the Duchess, who is considering legal action.
Royal author Phil Dampier said: “While we’re unsure of the real reason behind the rift, if Kate is cutting out friends just because of rumours, it’s worrying.
"She has already reportedly fallen out with her sister-in-law Meghan so to lose a lot of close friends as well will make it very difficult for her and William.
“In years gone by the royals and aristocrats tended to have a casual attitude to affairs and joked: ‘It’s alright as long as you don’t frighten the horses!’
“Kate is a middle-class girl who probably feels very differently.
“She probably has an insecurity because she split up with William once before.
“But as far as I know they are happily married and are even trying for a fourth child.”
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