Prince William was left in "a state of disbelief" after Catherine, Princess of Wales was diagnosed with cancer

In a new book, William and Catherine: The Monarchy's New Era: The Inside Story, a friend told author Russell Myers about the prince's reaction: "It was like being hit by a bus, sudden, brutal, and completely disorienting. One moment life was normal, and the next, everything changed. He worships her, truly. She's his world, and when the diagnosis came, it was as if the ground beneath him vanished.

"He talked about the rug being pulled, but it was more than that, it was heartbreak, fear and helplessness all at once. Watching him go through it was deeply emotional. You could see it in his eyes; in the way he held himself. But through it all, his devotion to her never wavered. He's been by her side every step, and the depth of his devotion is something that stays with you.

"It's love in its rawest, most powerful form."

Despite their own shock, the couple's priority was to make sure their three children, Prince George, 12, Princess Charlotte, 10, and seven-year-old Prince Louis felt secure and supported.

In an excerpt from the book published by the Sunday Mirror newspaper, an aide revealed: "[They] both very calmly told the children what was going on and how long Catherine would need to be away for, but explained other than that everything would continue as normal and when she came home, she would need to rest up for a bit."

While in hospital, Catherine kept in touch with video calls as she listened to stories about the children's days and asking if "Papa" had cooked for them while she was away.

The princess' parents and siblings were regular visitors to the family home, while William took over school drop-offs and lots of playdates were organised, as well as weekend visits to the children's grandparents, Michael and Carole Middleton.

A friend said: "They were both very conscious of protecting [the kids] as much as possible.

"Catherine's admiration for the way in which William took on all of that, keeping the children entertained and occupied, being constantly sanguine even when he had the weight of the world on his shoulders, allowing her to concentrate on her treatment and recovery, is something she will be eternally grateful for."