The monarch, 75, who has been mostly staying at Sandringham since his first round of treatment, held his weekly face-to-face audience with Rishi Sunak at Buckingham Palace on Tuesday.
The King says he has been “reduced to tears” by the “many wonderful messages and cards” he has received since his cancer diagnosis.
The monarch, 75, was speaking to Rishi Sunak in his first face-to-face audience with the prime minister since Buckingham Palace announced he had a “form of cancer”.
“I’ve had so many wonderful messages and cards, it has reduced me to tears most of the time,” the King told Mr Sunak.
The PM said it was “very nice to see you”, prompting the King to joke: “A bit of a gap… I’m afraid.”
“I hear there has been a lot more potential attention on those main, wonderful cancer charities, many of which I have been a patron of for years,” he added.
Mr Sunak replied: “They do a lot of wonderful work.”
The King had been mostly staying at Sandringham, in Norfolk, following his first round of cancer treatment in London last week.
On Sunday, the King was spotted smiling and waving to onlookers before attending a service at St Mary Magdalene Church.
Buckingham Palace announced on 5 February that His Majesty had been diagnosed with a form of cancer while undergoing a procedure for an enlarged prostate.
The palace did not release details about the type of cancer or the type of treatment.
Earlier this month, at an event at Salisbury Cathedral, the Queen spoke about her husband’s diagnosis for the first time.
She told Ben Abbott, a critical care paramedic: “He is doing extremely well under the circumstances. He is very touched by all of the letters and messages the public have been sending from everywhere. That’s very cheering.”
The announcement about his cancer was followed by a rare return to the UK for the King’s youngest son, Prince Harry, who flew over from the US and visited his father.