Prime Minister Rishi Sunak has said the whole country will be wishing the King well.
Writing on X, he says: "Wishing His Majesty a full and speedy recovery.
"I have no doubt he'll be back to full strength in no time and I know the whole country will be wishing him well."
King Charles has been diagnosed with a form of cancer, says Buckingham Palace.
It is not prostate cancer but was discovered during his recent treatment for an enlarged prostate.
The type of cancer has not been revealed, but according to a palace statement, the King began "regular treatments" on Monday.
Buckingham Palace says the King "remains wholly positive about his treatment and looks forward to returning to full public duty as soon as possible".
A little more than a week ago, the King had hospital treatment for an enlarged prostate, from which his wife Queen Camilla said afterwards he was recovering well.
The procedure was carried out at the London Clinic private hospital, where Catherine, the Princess of Wales had surgery the same week.
At the time, Buckingham Palace said King Charles had made his health problem public as a way of sending a message to other men to get their prostates checked. He thanked those who had "sent their good wishes".
The Palace says the King's cancer is not of the prostate - that a "separate issue of concern was noted" during his treatment and that subsequent tests identified a form of cancer.
You can read the statement in full in our last post.
