The 75-year-old monarch, still undergoing treatment which Buckingham Palace earlier said has been rated as “very encouraging” by his medical team, was also named Cancer Research UK's new patron as he joined wife Queen Camilla at the Macmillan Cancer Centre at University College Hospital. Smiling and cheerful Charles and Camilla spoke to staff and patients at the centre where the King was overheard saying it was important to get enough people diagnosed early.

“This visit will be the first in a number of external engagements His Majesty will undertake in the weeks ahead,” Buckingham Palace has said.

“His Majesty’s treatment programme will continue, but doctors are sufficiently pleased with the progress made so far that the King is now able to resume a number of public-facing duties. Forthcoming engagements will be adapted where necessary to minimise any risks to His Majesty’s continued recovery,” the palace said.

The royals also handed out presents to children and received some flowers in return. The King, who is also patron of Macmillan Cancer Support, said he was "much better” when people shouted out to ask how he was feeling.

The royal couple were introduced to clinicians to hear more about all the new technology at the hospital, including CT scanners which are being used to help with the early detection of cancer.

The King met Cancer Research UK’s chief clinician, Professor Charlie Swanton, who has led a project called TRACERx – a collaboration with 250 researchers and clinicians based at 19 centres across the UK, to investigate how lung cancer evolves.