King was “brought to tears” by cancer support messages sent to him

King Charles said the messages of support “brought me to tears”, as he was seen returning to official duties for the first time since his cancer diagnosis was revealed.

He was meeting Prime Minister Rishi Sunak, who said: “It's great to see you looking so good.”

“Everything is done with mirrors,” the king joked.

The King told Mr Sunak he had been deeply touched by all the cards and well wishes sent to him.

The King shook hands with Mr Sunak as he arrived to attend Buckingham Palace on Wednesday afternoon, and both were no doubt aware of the public scrutiny of the King's health.

The King appeared relaxed and wearing a blue striped suit, and sat with the Prime Minister in the ornately decorated audience room, surrounded by pictures of his royal relations and Canaletto's painting of the River Thames.

While receiving treatment, the king has stopped making public visits, but continues his duties as head of state, such as this regular meeting with the prime minister.

Mr Sunak was photographed arriving and the King told him how much he appreciated all the letters he had received and his admiration for cancer charities.

“I received so many wonderful letters and cards, it made me cry most of the time,” the king said.

But the rest of the meeting with the Prime Minister, which lasted about 45 minutes, was secret as usual.

The king was previously seen heading to church in Sandringham and driving his car to and from Clarence House in London, but this was his first sighting in such an official place since his cancer diagnosis was announced more than two weeks ago.

Earlier in the afternoon, the king attended the monthly meeting of the Privy Council, a body of the king's advisers, made up mostly of senior politicians from the House of Commons and the House of Lords.

Since being diagnosed with an unspecified type of cancer, the king has been receiving treatment as an outpatient.

He continued with the administrative side of his role, such as handling government papers in their red boxes and signing documents.

But he retreated from the tour of royal visits, where he usually met the public or attended events.

Wednesday's meeting with Sunak was a secret meeting between the king and the prime minister, usually held every week.

The meeting is a traditional opportunity for the politically neutral monarch to talk about current affairs of state with the sitting prime minister.

Former prime ministers spoke of these meetings as an opportunity to talk about ideas outside the usual partisan political pressures.

Edward Heath, the Conservative Prime Minister in the 1970s, said of his meetings with Queen Elizabeth II: “It was always a relief to be able to discuss everything with someone, knowing full well that there was not the slightest danger of any information leaking out.”

Labour's Harold Wilson, who was said to have had a good relationship with the late Queen, held longer meetings, sometimes extending to two hours.