King Charles' team is reportedly planning the cancer-battling monarch's summer diary

But a source has now told People he is "positive" about his cancer treatment, with his staff turning to his summer schedule as there has been "amplified confidence" about his condition in light of the sophisticated treatment he's receiving.

The insider added: "(The king is) progressing well. He is positive, the doctors are optimistic."

They added Charles has not been "operationally constrained" so far and Buckingham Palace is planning for "a summer diary".

But it is understood his staff will not commit to any engagements and will rely on "medical advice nearer the time".

Charles' cancer diagnosis was revealed on 5 February, and summer engagements that will go ahead include Trooping the Colour - the annual public celebration of the monarch's birthday, which will take place on 15 June.

The source added the king may build up to such events by increasing the numbers of people he meets indoors.

A second insider added Charles' team is "planning as usual", saying: "One thing that has been wholly undiminished is his appetite for work."

Charles last week travelled to Buckingham Palace from Windsor Castle to meet with Korean War veterans to mark the 70th anniversary of the conflict.

And late in February, the monarch undertook his first face-to-face meeting with British Prime Minister Rishi Sunak since revealing his cancer news.

Charles' nephew Peter Phillips, 46, has said his uncle is keen to get back to work following his diagnosis, telling Sky News Australia the king is in "good spirits" but also "frustrated" at the limited amount of work he's able to do.

He added: "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.

"He is always pushing, his staff and everybody and his doctors and nurses to be able to say, 'Actually, can I do this? Can I do that?' " Peter continued. "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."