Prince William and Princess Kate join King Charles with unexpected birthday message to Prince Harry

harry, kate and william
(Getty Images)

The Prince and Princess of Wales joined King Charles in sending an unexpected personal message to Prince Harry to mark his 40th birthday on Sunday.

In a post shared to their Instagram Stories, Prince William and Princess Kate wrote: "Wishing a Happy 40th Birthday to the Duke of Sussex."

Alongside their message, they included a joyous snapshot of Harry beaming for the camera in a sharp grey suit. The image also featured in an earlier tribute shared by King Charles. In a touching statement, he wrote: "Wishing The Duke of Sussex a very happy 40th birthday today!" alongside a cake emoji.

It is the first time the royal family has shared a public message for Harry since wishing him a happy birthday in 2021.

Harry, who lives in California with his wife Meghan Markle and their two children, has had a somewhat strained relationship with his father ever since he stepped down as a senior royal in 2020.

Charles, William and Harry at 2014 Invictus Games
King Charles with his sons Prince William and Prince Harry (Getty Images)

Following news of Charles's cancer diagnosis earlier this year, however, Harry travelled across the pond to reunite with his father at Clarence House. The pair were also reunited at the monarch's coronation last May.

Royal fans flocked to the comments section to share their thoughts, with one writing: "A very happy birthday to Prince Harry!" while a second chimed in: "Such a great photo and that smile! Wishing The Duke of Sussex a very Happy 40th Birthday!!!" and a third added: "Gorgeous pic! Happy birthday Harry!"

Prince Harry waving at Invictus tenth anniversary
Harry stepped down as a senior royal in 2020 (Getty Images)

In birthday plans exclusively revealed to HELLO!, the Duke will mark his big day on 15 September with a party for family and friends in Montecito, California, at the home he shares with his wife Meghan and their children.

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He will also enjoy a weekend away in the mountains with some of his closest friends.

Ahead of his milestone birthday, the father-of-two opened up about his role as a father and how it's become one of his "greatest joys" in life.

mother holding baby on sofa
Harry shares two children with his wife Meghan Markle (Getty Images)

"Becoming a father of two incredibly kind and funny kids has given me a fresh perspective on life, as well as sharpening my focus in all my work," Harry shared in a statement to the BBC via his spokesperson.

"Being a dad is one of life's greatest joys and has only made me more driven and more committed to making this world a better place."

Prince Harry and Meghan waving from carriage on wedding day
The couple tied the knot in 2018 (Getty Images)

Harry, who stepped down as a senior royal in 2020, also told the BBC: "I was anxious about 30, I'm excited about 40.

"Whatever the age, my mission is to continue showing up and doing good in the world".