The Prince and Princess of Wales were among the royal family members to send their heartfelt wishes to the Princess Royal on her 73rd birthday on Tuesday.
Prince William and Kate, both 41, reshared King Charles and Queen Camilla's tribute to Princess Anne on Instagram Stories and added their own message.
"Happy Birthday to The Princess Royal," the couple captioned the image.
The King's official social media accounts posted a previously unseen photo of Charles and Anne, taken on the day of the coronation in May.
Anne, dressed in military uniform, can be seen smiling at her older brother, Charles, who is wearing his Robes of Estate and the Imperial State Crown.
The Princess Royal was given an important role at the Westminster Abbey service as the King's Gold-Stick-in-Waiting, who is entrusted with the monarch's personal safety.
Anne rode horseback beside Charles and Camilla who returned to Buckingham Palace in the Gold State Coach following the coronation service.
A second photo shared by the King's social media account showed Charles as a young boy with his baby sister Anne in July 1951.
The young prince, then two, was pictured in his pram wearing a button-up coat, wrapped up under a soldier-printed blanket. In the sweet snap, Charles holds his baby sister Anne's hand as she sits in a pram next to her brother.
The little princess, who was then aged around 11 months, is wearing a cream coat with a frilled collar, and a knitted bonnet.
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The Princess Royal, who typically has a busy schedule of public engagements, is currently taking a break from her duties.
She is expected to be among the family members to join King Charles and Queen Camilla at Balmoral Castle during their summer break in Scotland.
The Prince and Princess of Wales and their three children, Prince George, Princess Charlotte and Prince Louis, are also likely to join the family reunion.
William and Kate traditionally spend some of the school holidays at their Norfolk residence, Anmer Hall. Last weekend, it emerged that the Princess attended a festival at nearby Houghton Hall – the home of the couple's friends, David Rocksavage and Rose Hanbury, formally known as the Marquess and Marchioness of Cholmondeley.