King Charles has felt "frustrated" amid his cancer treatment

Speaking to Sky News Australia, Peter shared: "He's in good spirits.

"I think, ultimately, he's hugely frustrated. He's frustrated that he can't get on and do everything that he wants to be able to do.

"But he is very pragmatic (and) he understands that there's a period of time that he really needs to focus on himself."

Charles is keen to return to some of his royal duties - even though he's been advised against it.

Peter said: "He is always pushing, his staff and everybody and his doctors and nurses to be able to say 'actually come on, you know, can I do this? Can I do that?'

"So the overriding message would be that he's obviously very keen to get back to a form of normality.

"And he's probably frustrated that, recovery is taking a little longer than probably he would want it to."

Charles' cancer diagnosis was publicly revealed on February 5.

A statement from Buckingham Palace read: "During The King's recent hospital procedure for benign prostate enlargement, a separate issue of concern was noted.

"Subsequent diagnostic tests have identified a form of cancer.

"His Majesty has today commenced a schedule of regular treatments, during which time he has been advised by doctors to postpone public-facing duties.

"Throughout this period, His Majesty will continue to undertake State business and official paperwork as usual.

"The King is grateful to his medical team for their swift intervention, which was made possible thanks to his recent hospital procedure.

"He remains wholly positive about his treatment and looks forward to returning to full public duty as soon as possible.

"His Majesty has chosen to share his diagnosis to prevent speculation and in the hope it may assist public understanding for all those around the world who are affected by cancer."