As her first cousin, Princess Alexandra is often seen close to Queen Elizabeth and has been referred to as her best friend.

Alexandra often accompanies the Queen to official events, including sitting next to Her Majesty in the royal carriage to Ascot.

She was also one of the bridesmaids at the Queen’s wedding to Prince Philip at Westminster Abbey, but Princess Alexandra ruffled feathers by referring to a young Prince Charles as having “gruesome habits”.

In 1952, Princess Alexandra wrote a letter to her brother Prince Edward, Duke of Kent, where she was explaining a conversation between herself and four-year-old Prince Charles.

 "Darling Eddie," she wrote. Princess Alexandra continued to explain that at a family gathering in Balmoral, they were playing a popular game known as "stone" when a guest named Martin "hit me quite accidentally on the mouth, which was extremely painful".

The next day, there was a blister after the injury, which required attention from a doctor.

 "So this morning Dr Middleton came and pierced it! Ho Ho. Charles was fascinated and insisted on watching. Gruesome child don't you think?"

The letter, which was dated September 21 1952, also described the ways in which Alexandra had been spending her time with the family, as well as Prince Philip.

 "We had a long discussion about black stockings. Too stupid. He is so sweet."

Princess Alexandra finished off the letter sweetly, wishing her brother to take care of himself.

 "Well look after yourself. With very, very many hugs & [kisses] from Georgie."

According to the Royal Family’s website, Alexandra has  "supported the Queen throughout her reign, through representing Her Majesty in the UK and overseas and attending events of national significance".

Alexandra is patron and president of over 100 organisations, including the Mental Health Foundation, St. Christopher’s Hospice and Queen Alexandra’s Royal Naval Nursing Service.

The letter that Princess Alexandra wrote to her brother has been in a private collection for 25 years, but was eventually sold at an auction in 2010. The letter sold for around $800.

At the time, Auctioneer Richard Davie said: "It is a rare letter with good references to the Royal Family."