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Mrs Rhodes, 91, passed away on Friday night following a short illness.The Queen is understood to have visited her trusted confidante at her Windsor home after she fell ill earlier this month. A royal insider said: “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.

GETTY The Queen and her oldest friend Margaret Rhodes

“One of the disadvantages of the Queen having such a long life is that she has had to say goodbye to so many dear friends and relatives. “It is such a sad loss for Her Majesty, Mrs Rhodes’s family and everyone in the royal household.” Many will have been familiar with Mrs Rhodes’s frequent appearances on documentaries about the Queen’s life, all of which were sanctioned by the 90-year-old monarch, who never gives interviews.

GETTY The Queen and a young Mrs Rhodes with Princess Margaret

Margaret Rhodes and Queen Elizabeth II through the years Tue, November 29, 2016 Queen Elizabeth II is mourning the death of her “best friend” and beloved first cousin, the Honourable Margaret Rhodes. Play slideshow 1 of 13

The cousins remained so close that they would take tea together every Sunday after church when the Queen was in Windsor. Born Margaret Elphinstone in 1925, Mrs Rhodes was the youngest daughter of Sidney Elphinstone, the 16th Lord Elphinstone and his wife Lady Mary Bowes-Lyon – sister to Queen Elizabeth the Queen Mother.

The Queen will be devastated to have lost her best friend Palace source

Being only 10 months apart in age, she and the then Princess Elizabeth were frequent playmates, riding hill ponies and picnicking together during summer holidays in Balmoral. In 1947, Mrs Rhodes was one of the eight bridesmaids at the future Queen’s wedding to Lieutenant Philip Mountbatten. She was also present at the Queen’s Coronation in 1953. During the Second World War Mrs Rhodes lived alongside the Royal Family at Buckingham Palace and Windsor Castle, and worked as a secretary for MI6.

GETTY Mrs Rhodes passed away Friday night following an illness at the age of 91

She married the writer Denys Gravenor Rhodes in 1950, with Princess Margaret as one of the bridesmaids, and the couple brought up two sons and two daughters in Devon. When Mr Rhodes was diagnosed with cancer, the Queen offered the couple the Garden House in Windsor Great Park so they could be closer to London for treatment. Mr Rhodes died in 1981 and his widow remained there. She once described the gesture as one of the Queen’s “many, many kindnesses”. In 1991, Mrs Rhodes became Woman of the Bedchamber, a mixture of lady-in-waiting and companion, to her aunt Queen Elizabeth until her death in 2002.