The Balmoral challenge passed by Princess Diana and Carole Middleton that Keir and Victoria Starmer may face

Princess Diana, Carole and Michael Middleton and the Princess of Wales are just three members of the upper echelons to undergo the 'Balmoral test' when they were invited to the royal's Highland abode for the first time.

The "Balmoral test," though not an official test, has been experienced not only by potential partners and in-laws of the royal family but also by the likes of politicians and their spouses, such as Tony and Cherie Blair, Boris and Carrie Johnson and famously Magaret Thatcher and her husband Denis. See Theresa May talking about her stay in the video below.

It may be Kier Starmer and his wife Victoria's turn to stay at the Scottish castle as following tradition, they are likely to be invited to Balmoral by the King during the Parliament summer recess.

While the "Balmoral test" may not be exactly as portrayed on The Crown, those invited to the Scottish estate are expected to know how to navigate the royal family's favourite sport: hunting, as well as enjoy long lunches and Highland walks in often questionable weather. 

© Tim Graham

Princess Diana famously caused a storm when she paid her first visit to Balmoral and passed with flying colours, as did Carole and Michael Middleton. In fact, Carole was even driven back from lunch by Queen Elizabeth II herself, according to The Telegraph.

As for Kier's predecessors, David Cameron once said there was not much "chillaxing" at Balmoral, with the royals spending their time on outdoor pursuits. 

The Princess of Wales made her first visit to Balmoral in 2009, two years before she married Prince William, and one year before her parents were first invited for a shooting weekend at the Scottish castle.

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Things must have gone well, as they were welcomed back in 2016 when Carole was seen being driven by Her Majesty on the way back from a grouse shoot together.

Balmoral holds a special place in the royals' hearts. Not only was it the late Queen's favourite residence, but her grandchildren have often spoken about how special the place is to them.

© X / @KensingtonRoyal

Princess Beatrice once said, "It's the most beautiful place on earth," adding that her late grandmother also loved visiting.

"I think Granny is happiest there. I think she really, really loves the Highlands... Walks, picnics, dogs—a lot of dogs, there's always dogs! And people coming in and out all the time.

"Family-wise, we're all there, so it's a lovely base for Granny and Grandpa, for us to come and see them up there, where you just have room to breathe and run."