The Queen and Prince Philip have been married for more than 70 years, living under a level of scrutiny seen by few other couples in the worl.
But it appears they still have a couple of secrets up their sleeve that keep at least some parts of their relationship behind closed doors.
A new book by a royal insider reveals that Philip had an inscription carved into his wife-to-be’s wedding ring – and what is written is known only to the couple and the engraver.
In her new extensive biography on Philip, 99, Ingrid Seward reveals how he personalised the ring, made with a nugget of Welsh gold.
Seward writes: “She never takes it off and inside the ring is an inscription. No one knows what it says, other than the engraver, the Queen and her husband.”
Philip and the then Princess Elizabeth married in November 1947, not long after the Second World War had ended, when rationing was still going on and budgets were tight.
Sailor Philip, who was from a royal family of his own, had surprisingly little to offer, but was able to fashion an engagement ring for his fiancee from a tiara that belonged to his mother, Princess Alice.
The Duke of Edinburgh and the Queen now have four children and eight grandchildren.
And with Princess Eugenie announcing her pregnancy earlier this month, they are now set to welcome their ninth great-grandchild.
The duke has carried out thousands of royal engagements during his life, always two steps behind his wife, who referred to him as her “strength and stay”.
Seward, who has written more than a dozen royal biographies and is the editor-in-chief of Majesty magazine, also revealed the duke took a young Prince Charles and Princess Anne camping in the woods in Balmoral.
She wrote: “When they were at Balmoral, he would take both Charles and Anne out in his Land Rover with sleeping bags and cooking equipment and spend the night in one of the many cabins on the estate.
“They would cook sausages over a fire and spend the night in their sleeping bags in a bothy built in Queen Victoria’s day as a picnic hut.”
Praising Philip as a “remarkable man”, Seward said the best thing she had discovered about the duke while putting the book together was his early action on environmental issues.
“Philip was so forward thinking that he tackled all these problems that we are facing today and even this week with Sir David, but no one in the general public gives Philip any credit as they don’t know what he has done behind the scenes,” she told Yahoo UK.
Philip’s grandson Prince William mentioned the Duke of Edinburgh’s work in a clip of a documentary to be shown on ITV next week.
In the book, Seward notes that Philip “has long argued that the ‘greenhouse effect’ and global warming are threats to the future of mankind and has stated that the greatest threat the world faces is the ‘colossal increase in the human population”.
Prince Philip retired in 2017, and now spends most of his time living on the Sandringham Estate in Wood Farm, a cottage on the Norfolk estate.
He joined the Queen in Windsor in March as the UK went into lockdown and they stayed together in the castle for several months, thought to be the longest time they had had together in years.
They went to Balmoral for a shorter-than-usual summer break, but will be split up again when the Queen returns to Windsor, where she is said to be planning to stay instead of returning to Buckingham Palace.
Buckingham Palace declined to comment.
Prince Philip Revealed is released on 1 October.