Princess Anne celebrates her 73rd birthday today (15 August) and The King paid tribute to his younger sister by sharing a sweet throwback photo.

A recent shot from The King's coronation was posted alongside a black and white photograph on the Royal Family's Instagram account. This is the official social media page for The King and Queen, as well as for extended Royal Family members.

The childhood photo features both Prince Charles and Prince Anne as young children sitting side by side in prams, in the gardens of Clarence House. Prince Charles can be seen leaning over to squeeze his baby sister’s hand as little Princess Anne smiles in a ribbon-tied bonnet.

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A post alongside the image wished, “Wishing Her Royal Highness The Princess Royal a very Happy Birthday today!” It also explained that the photograph of Queen Elizabeth II's two eldest children was taken in the year 1951.

Also included in the post is a snap of the siblings laughing together during the Coronation Day on 6 May earlier this year. In it, Princess Anne is in full, formal military dress while King Charles wears his Coronation robes and crown.

Members of the public shared their best wishes for the Princess on her birthday by leaving messages on the post.

“Love this so much ❤️ always looking at each other with respect and love,” read one comment while another said, "What a lovely picture of His Majesty and Her Royal Highness, thanks for sharing! Happy birthday to The Princess Royal."