Prince William says protecting his family was the most important matter for him following the illnesses of his wife Kate and father King Charles, while revealing he will seek to modernise the monarchy when he becomes sovereign.

Speaking to Eugene Levy for the Canadian actor's travel show, the normally guarded British heir said he sometimes felt a bit "overwhelmed" by family matters and vowed to keep his children safe from any excessive intrusion from the media.

"Worry or stress around the family side of things that does overwhelm me quite a bit," the 43-year-old royal tells the Schitt's Creek star, saying he felt he could manage his role as heir to the throne.

"When it’s to do with family and things like that, then that’s where I start getting a bit overwhelmed - as I think most people would."

Last year both Kate and the king began treatment for cancer and while his wife is now in remission, he said it had been the hardest year he had ever had.

"Life is sent to test us as well and it definitely can be challenging at times, and being able to overcome that is what makes us who we are," he said. "I’m so proud of my wife and my father, for how they’ve handled all of last year. My children have managed brilliantly as well."

The unusually candid remarks from William came as he showed the actor around Windsor Castle, the royal home west of London where the king hosted U.S. President Donald Trump last month, and also took him to a local pub.

William spoke about life as a royal in the media spotlight and his vision for the future. He referred to the intense press coverage of the break-up of the marriage of his father and his late mother, Princess Diana, saying they were "in everything, literally everywhere".

"And if you let that creep in, the damage it can do to your family life is something that I vowed would never happen to my family," he said. "And so, I take a very strong line about where I think that line is and those who over step it, you know I’ll fight against."

He said he did not think much about becoming king, but acknowledged he planned a shift in how he would perform the role, saying while tradition was important, he liked to question things.

"I think it’s safe to say that change is on my agenda. Change for good," he said. "That’s the bit that excites me, is the idea of being able to bring some change. Not overly radical change, but changes that I think that need to happen."

The episode of "The Reluctant Traveller With Eugene Levy" featuring William is set to air on Apple TV+ on Friday (October 3).