King Charles has been admitted to hospital for treatment for an enlarged prostate, says Buckingham Palace.
The procedure will be carried out at the London Clinic private hospital, where Catherine, the Princess of Wales had surgery last week.
It is understood that the King visited the Princess of Wales in the hospital this morning, ahead of his own scheduled treatment.
It is not yet known how long King Charles will remain in hospital.
King Charles had made his problem public as a way of sending a message to other men to get their prostates checked.
Following the disclosure, the NHS website saw a surge in searches about enlarged prostates, an interest welcomed by doctors and charities.
A statement from Buckingham Palace said the King was “delighted to learn that his diagnosis is having a positive impact on public health awareness”.