Charles and Queen Camilla flew by helicopter to Buckingham Palace from their private Sandringham home, and were driven the short distance to nearby Clarence House.

It was announced last week that British monarch has a “form of cancer” and at the weekend he issued a statement praising “all those organisations which support cancer patients”.

Charles has postponed all public-facing duties, but is continuing with behind-the-scenes work on his red boxes of state papers.

The Queen has been carrying out her royal diary of engagements and last week said her husband was doing “extremely well under the circumstances”.

She said the King, who underwent his first bout of cancer treatment last week, had been “very touched” by all the messages of support he has received from the public.

The King and Queen arrived as downpours swept across London and Charles was photographed waving to well-wishers.

Charles’s diagnosis followed three nights in a private London hospital where he was treated for an enlarged prostate.

The Prince of Wales may step in to represent his father at events and it has been reported William and his family are spending half-term at their Norfolk home Anmer Hall, which is on the King’s Sandringham estate.