Prince William has paid tribute “to the most incredible wife and mother” in a very personal birthday message.

A new picture of Princess Kate has been released to mark her birthday (Image: Matt Porteous)

A new picture of Princess Kate has been released to mark her 43rd birthday today, alongside a rare personal message from her husband, Prince William.

The previously unseen image was taken in Windsor last summer by photographer Matt Porteous.

In the post, he paid tribute “to the most incredible wife and mother” and added, “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.”

Kate’s birthday is not usually marked by a message from the prince or the release of a new image. But it is understood that William wanted to mark the occasion following Kate’s “brutal” year receiving cancer treatment.

Earlier on Thursday, King Charles and Queen Camilla wished Kate a happy birthday on their social media channels by sharing a photo of the princess taken on Christmas Day.

The royal couple have adopted a new approach to social media over the past 12 months, choosing to share updates on their terms.

Kate shared her cancer diagnosis in a poignant video, and later revealed she had completed her course of chemotherapy in a short film featuring her family.

To mark Prince William’s birthday in June, the occassion was marked by a personal message from Kate, who wrote: “Happy birthday Papa, we all love you so much! Cx.”

Kate is expected to spend her 43rd birthday with her closest friends and family today.

Following her “dreadful” year receiving cancer treatment, Kate, Prince William and their three children will mark the occasion quietly at home.

She is set to spend the day with her closest friends, before enjoying an intimate evening meal with William, Prince George, 11, Princess Charlotte, nine, and Prince Louis, six, pictured.

Kate was admitted to hospital for “planned abdominal surgery” last year, just a week after her 42nd birthday. She spent two weeks at the London Clinic before returning home for further recovery.

However, doctors subsequently discovered cancer had been present and the Princess started a course of preventative chemotherapy at the end of February.

In September, Kate announced she had finished her course of treatment and would be easing back into royal duties. The diagnosis, which came weeks after King Charles revealed he had been diagnosed with cancer, took its toll on the family.

Asked how his year had been while visiting Cape Town, South Africa, in November, William said: “It’s been dreadful. It’s probably been the hardest year in my life.

“So, trying to get through everything else and keep everything on track has been really difficult.”

He added: “I’m so proud of my wife, I’m proud of my father, for handling the things that they have done. But from a personal family point of view, it’s been brutal.”