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” yesterday.
Buckingham Palace said the King “remains wholly positive about his treatment and looks forward to returning to full public duty as soon as possible”.
He will postpone his public engagements and it is expected other senior royals will help to stand in for him during his treatment.
No further details are being shared on the stage of cancer or a prognosis.
The King, 75, returned to London from Sandringham in Norfolk yesterday morning and the palace said he has commenced treatment as an outpatient.
Although he will pause his public events, the King will continue with his constitutional role as head of state, including paperwork and private meetings.
He was seen at a church service in Sandringham on Sunday, where he waved to crowds.
He had a prostate procedure at a private London hospital more than a week ago.
The King had chosen to go public about his prostate treatment, with the aim of encouraging more men to get prostate checks, the palace said at the time.
He was said to have been delighted to have raised awareness about the issue, with the NHS website reporting a surge in issues about prostate conditions.
For many types of cancer, the chance of getting it increases with age. UK figures suggest, on average each year, more than a third (36%) of new cancer cases were in people aged 75 and over.
