The King spoke of his “shock” at being diagnosed with cancer as he met fellow patients on his return to public-facing duties.
Charles candidly described the experience of being told he had the disease when he visited University College Hospital Macmillan Cancer Centre in central London, to learn about the latest treatments and chat to staff providing the pioneering care.
He also reassured those who asked about the state of his health, saying: “I’m well.”
David Probert, chief executive of University College London Hospitals NHS Foundation Trust, who hosted the visit, said: “His Majesty was full of great energy, he engaged with us really positively and as always was incredibly enthusiastic.”
The King, who looked relaxed and in good spirits, was joined on the visit by the Queen, and went on impromptu walkabouts, meeting staff who had stopped to catch a glimpse of the royal couple.
Charles sympathised with cancer patient Lesley Woodbridge as she underwent chemotherapy alongside many others in a day unit, telling the 63-year-old: “It’s always a bit of a shock, isn’t it, when they tell you?”
He added: “I’ve got to have my treatment this afternoon as well.”
The King has been receiving treatment for an undisclosed form of cancer since early February, and on Friday it was announced he would be returning to public-facing duties.
The development indicated the positive progress he is making after almost three months of cancer care as an outpatient.
Asked about his health by one patient during Tuesday’s visit, he replied: “I’m all right, thank you very much, not too bad.”
Asha Millen, 60, who is receiving chemotherapy for bone marrow cancer, chatted to Charles as he met patients in the chemotherapy day unit.
She said afterwards: “I said ‘How are you?’ and he said ‘I’m well’.”
Mrs Woodbridge, from Houghton Regis in Bedfordshire, was joined by her husband Roger, 64, who spoke to Camilla while Charles chatted to his wife.
He said: “We were talking about how we feel. I said to her ‘How do you feel?” … She just said ‘It’s just so difficult’ and we both agreed.”
The King held Mrs Woodbridge’s hand before he left.
Charles later asked one male patient whether he was using a cold cap – a device aimed at preventing hair loss during chemotherapy – and the King was heard to say it can “help”.
The King and Queen both asked patients they spoke to about side-effects, particularly loss of appetite and taste, and tiredness.
Camilla left her husband for a few minutes and made her way to side cubicles, where she spoke to patients about their treatment.
“There’s a real feeling of calm here,” she said to Judy Parkinson. “Everybody in the same boat, everyone can identify.”
Patting the 69-year-old’s arm as she left, the Queen said: “I wish you the very best of luck, it’s a huge pleasure to meet you.”
Mrs Parkinson, who laughed as she heard the moment described as “two queens having a chat”, said: “I like it when people speak to you eye to eye. That’s what my mum taught me, and that’s what she did. Those were genuine questions being asked.”
The visit was marked by the King being named as the new patron of Cancer Research UK, which will sit alongside his patronage of Macmillan Cancer Support.
During their tour, the King and Queen were introduced to experts working on a new melanoma vaccine and met radiologist Sally Coe, who showed them one of two CT scanners she and her team use to screen patients for lung cancer.
“The trouble is to get enough people early enough,” said the King.
After shaking hands with dozens of medical staff waiting in reception as he left, the King received a round of applause from the NHS workers.
David Probert, chief executive of University College London Hospitals NHS Foundation Trust, said: “His Majesty was full of great energy, he engaged with us really positively and as always was incredibly enthusiastic.”
Charles’s diary of forthcoming events will not be a full summer programme, and attendance will be announced nearer the time and “subject to doctors’ advice”, with “adaptations made where necessary to minimise risk” to the King’s recovery.
It is understood the warmer weather would allow events to be staged outside and lessen the risk posed by other people that many cancer patients face.