In a rare move, William on Thursday issued a heartfelt message on social media after a turbulent past year in which Kate was diagnosed with cancer.
He made a public declaration of his and their children's love for Kate, telling her "We love you" in the message signed "W".
View this post on Instagram A post shared by The Prince and Princess of Wales (@princeandprincessofwales)
It was accompanied by a new, unseen black and white picture of Kate taken by Matt Porteous, in Windsor, showing the princess looking relaxed and smiling with her hands in her jean pockets and wearing a gingham scarf.
"To the most incredible wife and mother," William wrote.
"The strength you've shown over the last year has been remarkable. George, Charlotte, Louis and I are so proud of you.
"Happy Birthday, Catherine. We love you. W"
The future queen is celebrating quietly at home in Windsor with William, with Prince George, Princess Charlotte and Prince Louis spending the day at school before a family celebration in the evening.
Kate has endured a challenging past 12 months during which she had abdominal surgery, was diagnosed with cancer and faced chemotherapy treatment.
Next week marks a year since the princess was admitted to hospital for an operation on January 16, just seven days after her 42nd birthday.
William described 2024 as a "brutal" and probably "the hardest year" of his life, with his father, the King, also diagnosed with cancer.
