In 2006, the late Queen made the unprecedented move of inviting an ‘unregistered’ girlfriend for the first time – Kate Middleton, pictured on December 1 of that year

The Queen, pictured at the 2006 Christmas Day service, usually only invited royal couples to Sandringham once they got married

Every year the Royal Family travels to Sandringham to celebrate Christmas, where they have a number of famous traditions – from the gift giving to the annual ‘weigh in’ after the feast

Christmas at Sandringham is run like a military operation with everything timetabled, from the guests’ arrival, to eating breakfast, walking the dogs, sitting down to lunch and retiring to bed.

There are dozens of long-running traditions, some more bizarre than others, which take place every year.

One of them, which is reportedly 100 years old, involves everyone eating Christmas dinner with the Queen being weighed before and after their meal, just to ensure that they have been fed enough.

But amid all of the splendour and feasting, the Royal Family get to spend some quality time together away from prying eyes, on the 20,000-acre Norfolk Estate.

The occasion is known to be where royals let their hair down and enjoy a messy game of football together, along with card and party games.

It was to everyone’s surprise then, when it got to the winter of 2006 and Kate Middleton turned down such an opportunity to mingle with the royals.

She met William in 2001 when they were both studying at St Andrews University in Scotland, and the couple had since been to a number of royal events together over the years.

Kate was among the crowd watching William at his passing out parade at Sandhurst on December 15.

But tradition dictated that before being married into the Royal Family, attending big set-piece family occasions such as Christmas and Easter was off the cards.

Although Buckingham Palace courtiers later insisted the Queen could invite whoever she chooses to the intimate church service and family lunch, it is thought no one up until that point had ever been invited if they were not a permanent member of the family.

Kate had known William since they met at university in 2001 and the couple had since been to a number of royal events together over the years. Pictured at an old boys match at Eton College on March 18, 2006

Harry’s girlfriend at the time Chelsy Davy and Kate Middleton watch their boyfriends play in a charity polo match at the Beaufort Polo Club on July 29

Kate was among the crowd watching William at his passing out parade at Sandhurst on December 15

Queen Elizabeth II passes her grandson Prince William, as she inspects graduates during a passing out parade at the Royal Military Academy Sandhurst on December 15, 2006

Kate Middleton, with her parents Carole and Michael, at the Sovereign’s Parade at Sandhurst Military Academy for the passing-out parade of her boyfriend Prince William

Although Kate was an established girlfriend at that stage, she still chose not to come to Sandringham

Prince William arrives at church for the traditional Christmas Day service in Sandringham in 2006

Princesses Beatrice (left) and Eugenie, daughters of Prince Andrew, arrive at St Mary Magdalene church on the Sandringham estate on December 25, 2006

Queen Elizabeth II smiles at her great niece, four-year-old Margarita Armstrong-Jones in 2006

Princess Diana was not invited in 1980, two months before her engagement to Charles was announced.

In his biography Battle Of Brothers, historian Robert Lacey wrote: ‘By 2006, the couple had been dating seriously for the best part of five years.

‘Yet when William invited Kate to join him that year at Sandringham for the Royal Family’s traditional Christmas lunch, she refused.

‘It was the first time the Queen had extended such an invitation to an unregistered “girlfriend”, but Kate had her own take on that break with tradition: she would go to Sandringham on Christmas Day only when she was engaged and had a ring to prove it.’

The strong-willed ‘no’ from Kate showed she was willing to abide by tradition and wait until the time was right. She was not prepared to stamp over history in her rush to marry into the Royal Family.

So what did Kate do instead of spending Christmas at the world-famous Sandringham? She chose to travel to Scotland to spend the holiday with her parents and family.

A friend said at the time: ‘Carole, Kate’s mother, wants it to be a week-long celebration. Although Kate isn’t a wild party girl, her parents love entertaining and really know how to throw a bash.

Queen Elizabeth II is presented with flowers from young children after attending the traditional Christmas Day church service in 2006

After Kate turned the invitation down, the lovesick Prince William seemingly kept in touch with her by mobile phone while out hunting

Instead of spending Christmas at the world-famous Sandringham Estate, Kate chose to travel to Scotland to spend the holiday with her parents and family. Pictured with her mother Carole on November 10, 2006

Kate Middleton leaving the Embassy Club in London on December 20, 2006

‘They have rented a farmhouse in Perthshire and have arranged several days of shooting and big family dinners followed by nights of drinking around a big log fire.’

According to friends, Kate wanted to spend some time with her family following the recent deaths of her two grandmothers.

‘Kate was incredibly saddened to lose both her grandmothers and this will be the first time the family will have gathered since their deaths,’ the friend said. ‘It’s a bit of a Middleton Christmas and there’s no way Kate would not be part of it.’

But it appeared William seemingly regretted her decision, and on the traditional royal shooting party over the festive season, the prince was pictured glued to his phone.

The lovesick future king was in a world of his own, keeping his gun tucked under his arm but his mobile phone firmly in his hand.

As his uncles and grandfather enthusiastically took aim and proudly collected their kill, the then 24-year-old was texting (presumably Kate) furiously.

And Kate stuck to the values she displayed in 2006, taking until 2011 to eventually accept the invite to spend Christmas at Sandringham – eight months after marrying William at Westminster Abbey.

Despite having spent time with the Royal Family for over ten years, the then-Duchess of Cambridge later revealed she was nervous about her festive season with the in-laws.

On Christmas Day 2011, Kate showed no signs of nerves when she made her debut on the walk to the church service at St Mary Magdalene Church on Sandringham

Speaking on the 2016 ITV documentary, Our Queen At Ninety, she said: ‘I can remember being at Sandringham, for the first time, at Christmas.

‘And I was worried what to give the Queen as her Christmas present. I was thinking: “Gosh, what should I give her?”

‘I thought: “I’ll make her something.” Which could have gone horribly wrong. But I decided to make my granny’s recipe of chutney. I was slightly worried about it, but I noticed the next day that it was on the table.

‘I think such a simple gesture went such a long way for me and I’ve noticed since she’s done that on lots of occasions, and I think it just shows her thoughtfulness, really, and her care in looking after everybody.’

On Christmas Day 2011, Kate showed no signs of nerves when she made her debut on the walk to the church service at St Mary Magdalene Church on Sandringham. She continues to play a leading role in every royal Christmas.