London, UK - The UK royal family's Kate Middleton took to social media to honor Prince William on Father's Day as she continues to undergo treatment for cancer.

"We love you, Papa. Happy Father’s Day," the Princess of Wales (42) wrote to William (41) on Instagram Sunday, adding the initials "G, C & L" – which stand for their children George (10), Charlotte (9), and Louis (6).

In the snapshot, the father and his three royal children stand arm in arm on the beach with their backs to the camera, looking out to the sea.

The photo credit says Kate took the picture sometime in 2024.

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The tribute has garnered more than 1.5 million likes on Instagram.

"Princess Catherine, what a well lovely composed, photo with magic hour light. You have a real talent in photography. Thank you for sharing," one fan gushed.

William also shared a throwback picture of himself and his father, King Charles III (75), to his and Kate's shared Instagram.

The photo shows the father-son duo playing ball in a meadow of flowers while the Prince of Wales is just a boy.

"Happy Father’s Day, Pa," William wrote.

The day before, William and Kate appeared together at the Trooping the Colour parade, the annual birthday celebration for King Charles III in London.

The couple and their children drove up in a carriage and then joined the family on the Buckingham Palace balcony.

It was the princess' first public appearance since revealing her cancer diagnosis in March.

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Since that time, Kate has been undergoing chemotherapy absent from the public eye.

King Charles III is also undergoing medical treatment after announcing his own cancer diagnosis earlier this year.

The monarch made his return to public duties in April.