Buckingham Palace announced on Friday that doctors were sufficiently pleased with the 75-year-old king's response to treatment for the unspecified form of cancer that he would be able to resume some public-facing engagements.
The first of these saw the king and his wife Queen Camilla visit the University College Hospital Macmillan Cancer Centre where he met patients and medical specialists.
The British monarch, who has also become the new Patron of the Cancer Research UK charity, looked cheerful, waving to those gathered outside on his arrival on Tuesday.
Charles' health issues began in January when it was revealed that he would be admitted to hospital to have a corrective procedure for a benign enlarged prostate.
The following month, the palace said tests had uncovered the presence of a "form of cancer", but gave no further details beyond saying it did not involve his prostate.
He has rested and undergone treatment for the disease since then, continuing with official state duties in private. He was well enough to greet well-wishers after an Easter church service at the end of March.
Although his diary will be carefully managed to minimise any risks to his health, the palace said he might attend some annual events such as Trooping the Colour military parade in June, as well as commemorations to mark the 80th anniversary of the World War II D-Day landings, also in June.
The Palace has confirmed Charles and Camilla will host a state visit by the Japanese Emperor Naruhito and his wife Empress Masako in late June.
Charles' illness came less than 18 months after he succeeded his mother Queen Elizabeth. His nephew Peter Phillips has said the monarch, who is know for his desire to keep busy, had found the limitation imposed by his treatment frustrating.
Also absent has been Charles' daughter-in-law Kate, wife of his son and heir Prince William, who is undergoing preventative chemotherapy after tests in the wake of major abdominal surgery revealed cancer had been present.
