His mother, Queen Elizabeth II, traditionally spent Yuletide at Sandringham, remaining on the Norfolk estate until February to mark the death of her father, King George VI, who died on February 6, 1952.

But King Charles III and Queen Camilla, who celebrated their 20th wedding anniversary this year, chose to spend Hogmanay at Birkhall, their favourite Royal residence.

It was at the Scottish lodge, set on the Balmoral Estate, that the couple was infamously snapped in 1990 after the then Prince of Wales injured his back during a polo match – Princess Diana was down in London with William and Harry.

It was where Prince Charles proposed to Camilla Parker Bowles: The couple announced their engagement on February 10, 2005, after Charles got down on one knee. ‘I’m just coming down to Earth,’ Camilla said.

It was there that the couple spent their honeymoon, after getting married in Windsor on April 9, 2005, and it was there that the couple celebrated their 20th wedding anniversary this year after their four-day state visit to Italy.

But King Charles III is not the only Royal to have a special bond with Birkhall: his parents, Queen Elizabeth II and Prince Philip; the Duke of Kent; and the Duke and Duchess of Edinburgh also honeymooned there.

And the Scottish Lodge isn’t just a luxurious love nest: It has been at the centre of some of the most explosive moments in Royal history.

Now the Channel 5 documentary Birkhall: A Private Royal Residence, which airs tomorrow (Saturday), reveals the story of the King and Queen’s favourite Royal residence.

Royal biographer Ingrid Seward, Editor in Chief of Majesty magazine, told the programme: ‘Charles loves it there, Birkhall is definitely his favourite home.’

Former BBC Royal correspondent Jennie Bond added: ‘I think it’s one of his favourite places to be with Camilla, curled up on a settee – well probably she curls up, he probably doesn’t curl up – with a glass of whisky by a log fire.’

Birkhall sits deep within the Balmoral estate, just six miles from the Firm’s beloved Balmoral Castle. It’s an 18th-century stone farmhouse set in the shadow of the mountain Lochnagar.

With just three floors, it is small for a royal palace – more ‘beloved family home’ than ‘regal royal residence’. But it’s been a place for the Royals to escape to for decades.

Built in 1715 by Captain Charles Gordon of Abergeldie and his wife Rachel, the secluded house in the Highlands was discovered by Queen Victoria’s husband, Prince Albert, in 1848 when he took the lease on the Balmoral estate.

He thought Birkhall would make the perfect country retreat for his eldest son, Bertie, the future King Edward VII but he stayed there only once finding it too small and pokey.

Historian and broadcaster Wesley Kerr said: ‘It’s a sweet idea that it was for the Prince of Wales, but it was never going to be good enough for him.’

On the other hand, Victoria and Albert loved the house and turned it into the equivalent of a Victorian Airbnb for their friends and family.

However, it was not until the 1930s that the Scottish Highlands became the Royal’s favoured holiday destination – George VI and the Queen Mother, then Duke and Duchess of York, chose Birkhall for their annual summer vacation.

Both Princess Elizabeth and Princess Margaret loved the house, which the future Queen described as ‘the nicest place in the world’ and would cycle into the local village of Ballater to buy sweets.

The two sisters, then aged 13 and nine, spent the first four months of World War II at Birkhall, after being evacuated under Operation Pied Piper.

When George VI died in 1952, the Queen Mother retreated to Birkhall until Sir Winston Churchill drove over to Birkhall for a private visit and persuaded her to resume public life.

‘That meeting is very significant,’ adds Kerr, ‘and actually we got another 50 years of public service out of Queen Elizabeth the Queen Mother.’

The Queen Mum immediately drew up plans to renovate the house, installing a new wing with four new bedrooms and bathrooms, and French windows opening up onto the gardens.

She even cheekily put eight carriage and grandfather clocks in the dining room, to wind up guests as they all chimed at different times.

‘This apparently used to send her guests crazy because you just get into a conversation,’ said Bond. 

‘Then ding, ding, ding, ding, all over the place. But I think she thoroughly enjoyed her guests’ discomfort.’

Her sense of humour was also displayed when the architects, led by Arthur Penn, forgot to put a lavatory on the downstairs floor: She held an official opening of the water closet, which was fitted subsequently under the stairs.

‘The room was filled with flowers and the Queen Mother pulled the chain,’ said Kerr. 

‘And she said: “I declare this room Arthur’s seat.” 

‘Arthur’s Seat, of course, is a mountain in Scotland, but Arthur Penn had come up with the design.’

By the mid-1950s, Birkhall was one of the main stops on the Queen Mother’s annual Scottish tour. She embraced country living with gusto and was a keen fly fisher into her eighties.

She loved the card game Racing Demon – as long as she won. When the Duke of Atholl demanded a recount, he was never invited again.

‘He decided I’m really going to go for this,’ explained Seward. 

‘When it seemed that the Queen mother had won and he knew that he’d won, he demanded a recount. 

‘Then she lost, and she never invited him back again.’

‘Then she lost, and she never invited him back again.’

One of the Queen Mother’s favourite guests was her grandson, Prince Charles, who was the only person allowed to sleep in George VI’s bedroom.

‘She taught him about the finer things of life,’ said Seward, ‘opera, music, painting, drawing. Once he even dreamt that he ran away from Gordonstoun to join Granny.

‘At Birkhall, King George’s bedroom was on the ground floor, and it was kept as he left it. 

‘The only person who was ever allowed to sleep in there was Prince Charles. And that was a special honour given to him by his grandmother.’

When Charles married Princess Diana, Birkhall became a favourite holiday haunt for Princes William and Harry. But it soon became a place she associated with Camilla.

In 1990, nine years into his marriage, he recuperated at Birkhall after he was forced to cancel his engagements after injuring his back in a polo accident. But, instead of inviting his wife and sons to be with him, he chose Camilla.

‘He was photographed leaving Birkhall with Camilla,’ said Princess Diana’s Royal Protection Officer Ken Wharf. 

‘That was the scandal.’

After the Queen Mother died at Royal Lodge, Windsor, on March 30, 2002, Charles inherited Birkhall. It soon became a special place for him and Camilla.

‘I think when they land at Birkhall, it’s a case of really? Phew! Here we are’ added Bond. ‘We’re going to enjoy one another’s company.

‘We’re going to walk, and we’re going to sit, and we’re going to read like and have a little drink, and we’re going to just be together. And I think it is one of their very favourite places to be.’

Birkhall: A Private Royal Residence airs on Channel 5 at 8pm tomorrow (Saturday).