Queen Camilla joined her husband on the trip from Sandringham Estate, Norfolk, on the aircraft.
The Royal couple arrived back at Clarence House in London after spending a week at their 20,000-acre residency.
It is widely expected that he will undergo further treatments in the capital.
It was just this Saturday that His Majesty attended church in his first public outing since the nation was told of his diagnosis.
Walking outside St Mary Magdalene Church, he appeared cheerful and waved to members of the public.
But this morning, as their car was spotted near Clarence House, both Charles and Camilla looked low-spirited.
It was announced last week that the monarch has a ‘form of cancer’ and over the weekend he issued a statement praising ‘all 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.
Meanwhile, Camilla 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.
His return to London comes coincides with the release of the Duke and Duchess of Sussex’ new website Suxxes.com.
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