The King held an in-person audience with the Prime Minister on Wednesday, Buckingham Palace confirmed.
Charles, who is being treated for an undisclosed form of cancer, resumed his weekly face-to-face meeting with Rishi Sunak last week – his first since his diagnosis.
The monarch left Windsor Castle and returned to Clarence House on Tuesday, missing the service of thanksgiving for King Constantine of Greece.
The King and Mr Sunak met at nearby Buckingham Palace.
Although the King is continuing with duties behind the scenes, he has postponed all public-facing engagements while he undergoes treatment.
He told Mr Sunak last week how “wonderful” get well messages from the public had “reduced me to tears most of the time”.
The Queen, representing the King, led the royal family in St George’s Chapel in remembrance of the late Greek monarch Constantine.
The Prince of Wales, who was meant to be giving a reading at the service for his godfather, pulled out at the last minute because of an undisclosed personal matter.
The royal family was left in shock this week at news of the death of Thomas Kingston, the 45-year-old husband of Prince and Princess Michael of Kent’s daughter Lady Gabriella Kingston.
The King and Queen sent their “most heartfelt thoughts and prayers” to Lady Gabriella, who is known as Ella, and Mr Kingston’s parents and siblings.
Lady Gabriella is Charles’s second cousin. They are both great-grandchildren of King George V.