King Charles' very particular living conditions that lead to a 'constant battle' with Queen Camilla

Queen Camilla's younger sister, Annabel Elliot, has given an extremely rare insight into what Her Majesty and King Charles' relationship is really like behind closed doors.

Annabel, 74, speaks openly about her royal sister and brother-in-law's playful behaviour at home in an extract taken from royal expert Robert Hardman's new book, Charles III New King. New Court. The Inside Story.

Away from the regalities of royal life, it seems the King and Queen are just like any married couple, enjoying takeaways and taking part in petty arguments, such as the monarch's "compulsive window-opening" at Clarence House.

King Charles' penchant for frost in his palatial suites is arguably one of his more peculiar habits, which leads to "battles" with his wife who prefers her living spaces to be "warm and cosy".

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"There’s a constant battle about it," explains Annabel Elliot. "He will have opened it. She will creep in behind and shut it. So there’s a lot  of: 'Oh, darling, you shut the window.' 'Yes, I have, because we’re all freezing.' So a lot of banter goes on.'"

"If the King usually wins on that front, says his sister-in-law, Queen Camilla 'wins  most other things'," Robert concludes.

"If you ever wander around the royal buildings and are wondering where the King is, look for the flapping curtains," is the sentiment amongst his staff. Listen to Robert's full interview with HELLO! on the latest episode of A Right Royal Podcast below...

Yet King Charles' preference for cooler conditions may not be totally random. Studies show that hot, clammy offices and poor air quality could potentially reduce our brain power by up to 50%.

The King's disciplined work ethic is well documented, even down to his choice to view lunch as "a luxury that gets in the way of his work," according to royal commentator Gordon Raynor.

Lady Frederick Windsor also revealed in a recent interview with Tatler that the King "works all day long, has a quick supper and then disappears until about 4am to write letters."

While a warm and cosy office á la Queen Camilla's preference may be more inviting, a colder room with fresh, crisp air circulating through it is no doubt a must for supporting the King's razor sharp focus and long hours.

His Majesty's commitment to being a lifelong environmentalist has no doubt influenced his partiality for teeth-chattering conditions. He has been known to regularly hit net zero emissions at Buckingham Palace, and has recently turned down the temperature on the royal residence's secret swimming pool in a bid to further reduce energy consumption.

Writing in The Sunday Times last year, Julian Payne, his former communications secretary, recalled: "The King always has the windows wide open. A meeting at Birkhall, his home in the Highlands, in the middle of winter was not for the faint-hearted. I can remember more than one occasion when I thought I might have actually got frostbite while trying to write with a hand I could no longer feel."