The 's new had a hidden meaning behind its sartorial choice, according to a royal author.

issued a heartfelt message to his wife today praising her as the "most incredible wife and mother", saying "the strength you've shown over the last year has been remarkable".

The heir to the throne of his and their children's love for Kate, telling her "We love you" in the message signed "W".

William's message was accompanied by a new, unseen black and white taken by Matt Porteous, in Windsor, last summer.

The image showed the princess looking relaxed and smiling with her hands in her jean pockets and wearing a gingham scarf.

She had styled her hair down and was dressed in a dark blazer and a white collared shirt.

Kate could also be seen wearing her sapphire and diamond engagement ring, which used to belong to William's late mother, Princess Diana.

Now royal historian Dr Tessa Dunlop claimed the new picture carried a hidden meaning in how the younger generation of royals want to portray themselves to the public.

She told the : "Black and white, timeless and yet contemporary, the portrait photograph released to celebrate the 43rd birthday of the Princess of Wales is breathtaking, likewise the message that accompanies it.

"William is letting everyone know just how proud he is of his wife, and likewise what they have achieved together."

She added that the photo "deliberately" aimed to show Kate as an "everyday princess".

The author explained: "Released on their Instagram page, the birthday message is intended for a wide audience.

"Deliberately unshowy, a simple photo of Kate in jeans speaks to an everyday version of the princess.

"The family are sharing her inspirational story with us all, as the nation battles unprecedented mental and physical health issues in the year's bluest month, January."

Kate is said to be 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.

The future Queen has endured a challenging past 12 months which saw her undergo abdominal surgery, be diagnosed with cancer and face chemotherapy treatment.