As the Queen prepares to celebrate her Diamond Jubilee, one of her closest relatives and oldest friends has been allowed to give an unprecedented insight into the family life of the Royals. There have been hundreds of other books claiming to offer a glimpse behind Palace doors, but this is the first written by someone who is closely related to the Royal Family and has shared their lives – not only throughout the Queen's reign but also through that of her father, King George VI.

The Final Curtsey, by Her Majesty's cousin and childhood playmate the Honourable Margaret Rhodes, is being serialised exclusively by The Mail on Sunday, starting today.

It tells in enchanting detail the story of Mrs Rhodes's relationship with the Royal Family over eight decades. The book, illustrated with delightfully informal and never-before-seen pictures, has been written with the full knowledge of the Queen, who has read and approved parts of it.

The Queen and I: The Honourable Lady Margaret Rhodes sits resting with the Queen during a trek through deer-stalking area at Balmoral

Born in 1925, the youngest daughter of the 16th Lord Elphinstone and his wife Mary (nee Bowes-Lyon), the Honourable Margaret Rhodes has led an extraordinary life. She was the childhood playmate of her cousin, the Queen; a wartime MI6 operative; and Lady-in-Waiting to her aunt, the Queen Mother. Now, at the age of 86, she has written an enchanting autobiography that paints an unprecedentedly intimate portrait of the private world of the Royal Family...

The entertainer: The Queen Mother, sporting tartan and a sprig of heather in a jaunty hat, shares a joke with Margaret at the log cabin at Birkhall she named the Old Bull and Bush

A world away from state banquets: The Queen and Margaret enjoy a relaxed lunch at Glen Beg, Her Majesty's log cabin on the Balmoral estate - with the Queen's gin and Dubonnet perched on the side

Solitary moment: Prince Philip breaks into his sandwich box for a snack in the shade of a stone house built by Queen Victoria for Prince Albert at Balmoral