Prince William took royal fans by surprise yesterday as he issued a heartfelt public birthday message to his wife Princess Kate.

The Prince of Wales, 42, took to social media on Kate’s 43rd birthday with a black-and-white photograph of the princess in a casual jeans and shirt outfit.

However, the future King on Thursday did more than just wish his wife a happy birthday as he also addressed the tough year the family have faced with an emotional message.

He wrote: “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. Happy Birthday, Catherine. We love you.”

The statement varied greatly from those usually posted by members of the Royal Family online as it showed the prince and his emotional side, something that really came to light more than ever last year during Kate’s cancer battle.

Speaking on behalf of JeffBet, body language expert Darren Stanton said the statement also showed that William is not afraid to show his love for Kate.

He said: “For someone to make such an overt display of public affection, whether it’s physical or a statement on social media, it usually means they are confident to speak this openly behind closed doors.

“This is not the behaviour we’ve typically seen from older royals but it shows we’re in a new era. William clearly loves his wife, is forging his own path as the future King and isn’t afraid to show his emotions.”

He continued: “William’s tribute to Kate demonstrates the love and connection they already share and no doubt proves they’ve become more open and honest about their feelings after enduring such a challenge last year.”

It is believed Princess Kate would have spent her birthday at their home Adelaide Cottage in Windsor with her nearest and dearest family members.

After finishing her chemotherapy in September, the Princess of Wales has been recovering at home as she continues a gradual return to work, which is set to continue this year.