Kensington Palace posted the personal written message from William alongside a previously unseen picture of Catherine taken by Matt Porteous in Windsor last summer.

The black and white picture showed the princess looking relaxed and smiling, wearing jeans, a white shirt and a scarf.

It follows in the couple’s style of informal and relaxed pictures, without any traditional royal imagery.

The Royal Family also wished her a happy birthday.

The monarchy’s official media account shared a message on X saying: “Happy Birthday to The Princess of Wales!” along with a birthday cake emoji.

The post also had a photo of Catherine collecting flowers from wellwishers while attending church in Sandringham on Christmas Day.

That was the last time the Prince and Princess of Wales were seen together in public.

It is understood that Catherine is celebrating her birthday at home in Windsor, with William and their three children, Prince George, Princess Charlotte and Prince Louis.

William has previously described 2024 as probably “the hardest year” of his life, with his father, the King, also diagnosed with cancer.

Last month, the Prince and Princess of Wales issued a Christmas card showing a picture taken from the video released when Catherine announced the end of her chemotherapy.

The emotional video, with its emphasis on family and supporting each other in difficult times, was filmed in Norfolk in August.

It had the message from Catherine that “out of darkness can come light”.