While the palace has not specified the form of cancer, it said the 75-year-old monarch has started treatment and remains "wholly positive".

He will be postponing his public duties on medical advice but will continue with his duties as a Head of State such as essential paperwork and private meetings.

The statement noted that the King is grateful to his medical team for their swift intervention, which was made possible thanks to his recent hospital procedure.

The Palace released a new unseen portrait of the King to accompany the announcement, showing him in a formal pose in a suit and tie with his hands behind his back during his state visit to France in September.

The King is said to have informed both his sons, William and Harry, personally about his diagnosis and Prince William is in regular contact with his father. Meanwhile, sources said that Prince Harry, who lives in the US, has spoken to his father and plans to travel to the UK to see him in the coming days.

Charles became King in September 2022 when his mother Queen Elizabeth II died at the age of 96.

The King chose to go public about his cancer treatment as he had been a patron of several cancer-related charities when he was Prince of Wales, a spokesman said. He had also gone public about his prostate treatment, to encourage more men to get prostate checks.

The Royal Society of Medicine thanked the King for highlighting "how cancer is indiscriminate" and urged members of the public eligible for cancer screenings to make an appointment.

The King and Queen are scheduled to visit Canada in May, and Australia, New Zealand and Samoa for the Commonwealth Heads of Government Meeting in October. The Palace has yet to confirm whether the tours will go ahead, with no date suggested for the King's return to full public duties.