His Majesty was admitted to The London Clinic, a private hospital near Regent Street, on Friday.
It is the same hospital where the Princess of Wales is being cared for following an abdominal complaint.
The 75-year-old monarch took time to visit his daughter-in-law, who is on the 13th day of her hospital stay, after successfully undergoing surgery.
The Queen’s visit was the first time she had seen her husband since arriving with him on Friday morning.
Wearing a white blouse and black jumper, she left around three hours later, telling well-wishers that the King was “doing well” after the procedure.
Buckingham Palace would not confirm last night how long the King was expected to spend in hospital, though the average stay for those undergoing the procedure is around three days.
However, this marks the beginning of a month-long recovery period during which doctors recommend no strenuous activity or heavy lifting.
Charles has cancelled all official engagements while he recuperates, while Kate, 42, is not expected to carry out official engagements until after Easter.
The Prince of Wales is believed to have cleared his diary of official duties for the time being as well to support his wife and father.
The monarch was diagnosed with the benign condition on January 17 while staying at Birkhall, Aberdeenshire, after going for a check-up because he was experiencing symptoms.
He arrived back in London from Norfolk on Thursday afternoon.
Prime Minister Rishi Sunak wished Charles the “very best” for his treatment and a “speedy recovery” afterwards, a Downing Street spokeswoman said.
A Buckingham Palace spokesman previously said the King was “admitted to a London hospital for scheduled treatment”.
The spokesman added: “His Majesty would like to thank all those who have sent their good wishes over the past week and is delighted to learn that his diagnosis is having a positive impact on public health awareness.”
The King is understood to have wanted to share the news to encourage other men to get themselves checked.
