In a statement, Buckingham Palace said that the King had been diagnosed with cancer.
He was in the hospital for treatment, and the palace noted that he did not have prostate cancer, but it did not provide further details.
The 75-year-old King is expected to continue with his treatment.
According to a source, Prince Harry will be traveling to Britain to visit his father in the next few days.
The King informed both Prince Harry and Prince Charles about his condition.
The palace also said that the other members of the royal family, including the Duke of Edinburgh and the Princess Royal, were informed of the King’s health situation.
Prince William, who is the heir to the throne, is said to have been in regular contact with the King.
The monarch has started a regular treatment program, and he is receiving expert care from a team of doctors. But, due to his condition, the king has been asked to delay his public duties. He left his country estate at Sandringham and traveled to London to begin outpatient treatment.
The king spent the evening at his home in London, most likely staying in Clarence House.
He will continue with his regular royal duties, such as carrying out official papers and state business.
A palace spokesperson said that the king had been treated for a benign form of prostate enlargement.
During this course of treatment, a different issue of concern was noted.
This second condition will be treated appropriately.
The king was seen waving as he and the queen attended a church ceremony at Sandringham, his first appearance in public since he was released from a London clinic last week following his recent medical treatment.
He was admitted to the hospital for a prostate enlargement procedure.