This week, Buckingham Palace announced King Charles had been diagnosed with cancer and would postpone public appearances.
Charles, 75, had spent three nights in hospital last month after undergoing a corrective procedure for an enlarged prostate, when a separate issue of concern was discovered and identified by further tests as cancer.
The palace did not give any details about Charles' cancer but a royal spokesperson said that it was not prostate.
Earlier this week, Prince Harry flew into London from Los Angeles to see his father.
They reportedly met on Tuesday for less than an hour, before Charles and Queen Camilla flew by helicopter to the royal estate in Sandringham.
Harry then stayed at a London hotel overnight and flew back to the United States the next day.
He did not see his brother Prince William, who has this week returned to royal engagements for the first time following wife Kate Middleton's stay in hospital.
The King has started a schedule of regular treatments but has been advised to postpone his public-facing duties.
He will carry on working behind the scenes on state business and official papers.
"The King is grateful to his medical team for their swift intervention, which was made possible thanks to his recent hospital procedure," the Buckingham Palace statement said.