The royal couple selected two touching photos to accompany their message, the first of which showed William and Kate with the monarch as they showed her round the Duchess's Back to Nature Garden at the 2019 Chelsea Flower Show.
WATCH: The Queen's grandchildren and great-grandchildren lead the way at Easter service
The second image poignantly shows the Queen and her late husband Prince Philip posing with seven of their great-grandchildren. The monarch can be seen holding little Prince Louis on her lap, with Princess Charlotte sat in between the monarch and the Duke as she strikes a playful pose, and Prince George stood to the side.
Standing behind the sofa is Savannah Phillips, while her sister Isla appears alongside her cousins, Mia and Lena Tindall, Zara Tindall's children.
The Cambridges have shared a heartfelt tribute to the Queen on her 96th birthday
The sweet family photo was taken at Balmoral Castle in 2018 by keen photographer Kate, and shows a more relaxed side to the monarch and her late husband.
"Wishing Her Majesty The Queen a very happy 96th birthday today!" William and Kate wrote in the caption. "An inspiration to so many across the UK, the Commonwealth and the world, it's particularly special to be celebrating in this #PlatinumJubilee year."
A new photo of the monarch has been released to mark her milestone
In celebration of the Queen's birthday, a new photo has been released by the Palace, showing the monarch enjoying the spring sunshine as she holds the reins of two of her beloved ponies.
On Wednesday, she was flown by helicopter to Sandringham where she is marking her milestone birthday in private. The Queen is staying at Wood Farm, the modest cottage which the Duke of Edinburgh made his retirement base in his final years.
The Queen and her beloved husband were married for 73 years before his death
The royal couple spent many happy weekends there together over the years and the Queen will no doubt spend time reflecting on treasured memories.
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