The first cousin of her Majesty, who was interviewed many times by HELLO! magazine, was visited by the monarch at her home in Windsor after she reportedly became ill in November. Margaret was also a god daughter of King George VI and was a bridesmaid at the wedding of the Queen and Prince Philip.
She was the daughter of the 16th Lord Elphinstone and Lady Mary Bowes-Lyon, who was the sister of The Queen Mother, Queen Elizabeth. Margaret married writer Denys Gravenor Rhodes in 1950, where the late Princess Margaret acted as one of her bridesmaids, and has four children and one grandchild.
Buckingham Palace have confirmed that the Queen is aware and confirmed that it is a "private matter". A source told the Daily Mail: "The Queen will be devastated to have lost her best friend. She grew up with Mrs Rhodes and they had so much shared history.
They remained close throughout the Queen's reign and Her Majesty continued to be a regular visitor at Mrs Rhodes's Windsor home until very recently. It is such a sad loss for Her Majesty, Mrs Rhodes' family and everyone in the royal household. She was an extraordinary woman."
Mrs Rhodes was a friend of the Queen from a young age, and gave an insight into her early life. Speaking to host Christiane Amanpour on CNN International's Amanpour, she said: "The King and Queen in those days made an enormous effort to keep [the Queen and Princess Margaret's] childhood sort of sacrosanct. I mean, it was just a time for learning and enjoying. And I think that they have succeeded awfully well."
She also confirmed that the Queen had no plans to retire at the age of 80, telling the BBC: "I'm perfectly certain she will never retire as such. It's not like a normal job and, to the Queen, the vows that she made on Coronation Day are something so deep and so special that she wouldn't consider not continuing to fulfil those vows until she dies."