A former aide of Prince William has revealed how difficult it was for the first-in-line to the throne when he found out his wife Princess Kate had cancer last year.

After a planned abdominal surgery at the start of last year, the Princess of Wales, 43, then received a cancer diagnosis shortly afterwards.

While Prince William, 42, has since spoken out about his wife’s cancer, calling last year “brutal”, he has never revealed how he really felt in the moment when the diagnosis was first made.

Now the princess has finished chemotherapy and is in remission, a former aide of the couple has revealed how William really coped with the diagnosis.

In an interview with 60 Minutes Australia for a documentary called Where There’s a Will, Jason Knauf, a former chief executive of the Prince and Princess of Wales’ Royal Foundation, described the prince’s reaction to his wife's cancer in a heartbreaking six-word remark.

Knauf, who also once worked for the Duke and Duchess of Sussex, said the prince was the “lowest I have ever seen him”.

He also revealed how he heard the Princess of Wales was ill through a phone call.

He said in the documentary: “Within a couple of weeks, if you’re Prince William, you find out that both your wife and your father have cancer. I couldn’t believe it.

“It was awful, absolutely awful. It’s the lowest I’ve ever seen him (William).”

Knauf also said that the couple did not tell the world straight away as they were working out the best way to tell their three children Prince George, 11, Princess Charlotte, nine, and Prince Louis, six.

He said: “But the problem was that all this crazy conspiracy theory stuff kicked off in the background, online. ‘Was she really ill?’.

“But they didn’t want to say yet that she had cancer because they hadn’t told the children and they were still working through how to tell the children.”

After being diagnosed with cancer last year, Princess Kate stepped back from royal duty as she began chemotherapy, which came to an end in September. The princess then confirmed just last month that she is now in remission.