The King reassured fellow cancer patients about the state of his health when he told them “I’m well” as he returned to public-facing duties.

Charles looked relaxed and in good spirits during a visit to University College Hospital Macmillan Cancer Centre where he learnt about some of the latest treatments and sat down to chat to patients receiving chemotherapy.

The head of state was joined by Queen Camilla during the trip to the London-based medical institution and they went on a number of impromptu walkabouts, meeting staff who stopped to catch a glimpse of the couple.

The King sympathised with one cancer patient as she received her chemotherapy with many others in a day unit, telling Lesley Woodbridge, 63: “I’ve got to have my treatment this afternoon as well.”

Charles 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.

When quizzed about his health, he told one patient: “I’m all right thank you very much, not too bad.”

The development indicated the positive progress Charles is making after almost three months of cancer care as an outpatient.

Asha Millen, 60, who is receiving chemotherapy for bone marrow cancer chatted to the King as he met patients in the chemotherapy day unit.

She asked Charles about his own health, saying: “I said ‘how are you?’ and he said ‘I’m well’.”

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.