Kate Middleton cancer diagnosis gives major shock to King Charles

King Charles III, who's himself battling cancer, is "extremely concerned" about Princess Kate amid her health troubles, according to a royal expert.

Kate Middleton and her father-in-law King Charles are currently undergoing treatment for undisclosed forms of cancer.

Robert Hardman, author of Charles III: New King, New Court, tried to explain the monarch's real feelings about his beloved daughter-in -law, saying: "It's very clear that he [King Charles] is extremely concerned about her [Kate]."

The author went on explaining: "I think he wants to make sure as much as he can that if there's anything he can do, he will do, because for a very fit and healthy young mother, this is a different order of magnitude to someone having to deal with this in their 70s."

Hardman told Hello!: "I'm sure he's been a source of comfort for her in the same way she's probably been a source of comfort for him. I think it cannot but have brought them even closer together."

It was previously reported that the King visited Princess Kate in hospital while she was recovering from an abdominal surgery.

Following her operation, Kate and King remained close and even enjoyed a private lunch together at Windsor Castle. The meeting reportedly left the King feeling "very emotional" as he "thinks of Catherine as his daughter."

“He's always had a huge regard for his daughter-in-law. He thinks she's just wonderful. When people are in hospital, they don't really want a fuss. But the fact was: the King was there, she was there, and the Queen was there with him as well. And they did go around to check and reassure the Princess,” claimed the author.

"And we have to have great sympathy with the fact that they are undergoing an absolute shock of a lifetime, except they're having to undergo it with all of us talking about it."

King Charles is also undergoing cancer treatment but is remaining “positive” and “progressing well,” according to sources.