Prince William celebrates Kate Middleton's 43rd birthday with rare move as he declares his love

Princess of Wales Kate Middleton announces in a new video that she is focused on staying cancer-free after completing her treatment.

Prince William is celebrating his wife Kate Middleton on her 43rd birthday.

On Thursday, the Prince of Wales took to the couple’s social media and shared a previously unseen black-and-white portrait of the mother of three.

"To the most incredible wife and mother," William, 42, captioned the post. "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."

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"We love you. W," he closed the message.

People magazine noted that the Prince and Princess of Wales don’t typically write personal messages on their social media. It shared that the post is a rare public declaration of William’s love and support for his wife, who completed chemotherapy treatment in September.

According to the outlet, Matt Porteous took the photo in Windsor Castle last summer. It shows a relaxed Kate sporting a plaid scarf, a white T-shirt, and jeans paired with a blazer. 

The Princess of Wales, who was all smiles, completed the casual look with her sapphire and diamond engagement ring, which belonged to her late mother-in-law, Princess Diana.

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The Prince and Princess of Wales married in 2011. They share three children. (Chris Jackson/Getty Images)

British royal expert Hilary Fordwich told Fox News Digital Kate is spending the day at home at Adelaide Cottage, Windsor, alongside William and their children: Prince George, 11, Princess Charlotte, 9, and Prince Louis, 6.

"While usually still in Norfolk at their Anmer Hall home for Princess Catherine’s birthday, this year they returned early to Adelaide Cottage, so her children could be back to school," said Fordwich. "Ever the devoted and engaged parent, her children come first, so she is more concerned with their schooling than her birthday."

"To the most incredible wife and mother. The strength you’ve shown over the last year has been remarkable. George, Charlotte, Louis and I are so proud of you."

"Given her birthday falls today, on a school day, and although Prince William has professed not to be a particularly brilliant cook, he is known to make a great breakfast, which he may well have made for her this morning," Fordwich added.

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In September, Kate Middleton announced she was "cancer free" after completing chemotherapy treatments. (Will Warr)

In March 2024, Kate announced that she was undergoing treatment for cancer. In December, during her Christmas carol service, the princess admitted, "I didn’t know this year was going to be the year that I’ve just had."

Robert Hardman, author of "The Making of a King: Charles III and the Modern Monarchy," told People that Kate is slowly returning to royal duties while prioritizing her health.

WATCH: KATE MIDDLETON ANNOUNCES SHE HAS COMPLETED CHEMOTHERAPY TREATMENT

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"We are being encouraged not to look at this as a chapter that’s over, and then it's back to normal because cancer doesn’t work like that," said Hardman. "The details remain private, but there’s no getting over the reality that it’s an unpredictable disease."

"The clue is in her phrase 'one day at a time' – that’s how she’s going to approach it," Hardman shared, referring to her September announcement about completing chemotherapy. 

Stephanie Nolasco covers entertainment at Foxnews.com.