The previously unseen photo was taken on the day of Archie's royal christening in July and shows Harry holding his son - who wears the handmade replica of the official royal christening robe - while Prince Charles looks at the youngster adoringly.
Shared to the couple's Sussex Royal Instagram account, the caption says: 'Happy Birthday to His Royal Highness The Prince Of Wales - Sir, Pa, Grandpa!'
Prince Charles was one of the few guests to Archie's private christening, with he and wife Camilla, the Duchess of Cornwall, joining Prince William and Kate, Meghan's mother Doria Ragland and Princess Diana's sisters Lady Jane Fellowes and Lady Sarah McCorquodale for the special day in Windsor.
The Duke and Duchess of Cambridge also wished Charles a Happy Birthday, sharing a montage of pictures featuring Charles holding hands with his grandson Prince Louis, with son Prince William at an event and with both sons William and Harry together.
The heir to the throne turns 71 today. Last year, for his monumental 70th birthday a special image of Charles bouncing his grandson Prince George on his knee and joined by his immediate family - including Princess Charlotte. Prince Louis, the Cambridges and Sussexes - was shared by Clarence House.
For his birthday this year, Charles is spending it on a royal visit to India where he will focus on climate change, sustainability and connections between the UK and India.
On Wednesday, the royal was pictured with surprising guest Katy Perry at an event in Mumbai to mark the British Asian Trust, which aims to tackle poverty and inequality in South Asia.
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