The King has cancer and will be postponing his public duties while he receives treatment, Buckingham Palace announced on Monday.
The diagnosis came after a hospital procedure for a benign prostate enlargement revealed a second, more serious medical problem.
The King has begun regular treatment at a London hospital, but has not revealed what type of cancer he has, other than confirming it is not prostate cancer. He is taking time away from public engagements to minimise the risk to his health.
He was able to tell members of his family personally, with his younger son, the Duke of Sussex, now flying back to Britain to see him.
The Prince of Wales, who is already supporting his wife after her serious abdominal surgery, will this week resume public engagements, including conducting an investiture. He is in regular contact with his father.
The Queen is to continue her full programme of engagements as she maintains her support after visiting her husband every day in hospital during his prostate treatment.
The King is said to be “wholly positive about his treatment”, and “looks forward to returning to full public duty as soon as possible”.
A source added that the King was as optimistic “as can be”, having caught the cancer early and with specialist treatment available.
He will continue to undertake state duties, including his audiences with the Prime Minister, Privy Council meetings and the completion of paperwork in his red boxes.
Some meetings may be held virtually to minimise the risk of infection to the King during his treatment. There are no plans yet to appoint counsellors of state to deputise for him.
The news was announced by Buckingham Palace at 6pm on Monday, after the family – including the Princess Royal, the Duke of York and the Duke of Edinburgh – the Government, and leaders of the nations where the King is head of state were informed.
Asked about the form of cancer, a spokesman said: “No further details are being shared at this stage, except to confirm that His Majesty does not have prostate cancer.”
The King started his treatment as an outpatient on Monday and remained at home in London in the evening, a spokesman confirmed.
He added that, as patron of a number of cancer-related charities, the King had chosen to make his diagnosis public once treatment had begun.
“In this capacity, His Majesty has often spoken publicly in support of cancer patients, their loved ones and the wonderful health professionals who help care for them,” he added.
The King has now received well wishes from around the world. Joe Biden, the US president, said he was “concerned” about the diagnosis as foreign leaders wished him a swift recovery.
Mr Biden, who is six years older than the 75-year-old King, told reporters: “I’m concerned about him. Just heard about his diagnosis. I’ll be talking to him, God willing.”
In a formal statement, Mr Biden said: “Navigating a cancer diagnosis, treatment, and survivorship takes hope and absolute courage. Jill and I join the people of the United Kingdom in praying that His Majesty experiences a swift and full recovery.”
The King was most recently seen attending a church service on Sunday.
He looked weary, but smiled and waved to the public as he and the Queen walked to St Mary Magdalene Church in Sandringham, Norfolk.
The King was discharged from the London Clinic a week ago, after three nights in hospital following treatment for an enlarged prostate.
He underwent a corrective procedure for the benign condition and had already been planning to take up to a month off public duty as he recuperates.
Since then, the Queen has been the most senior member of the Royal family carrying out public engagements while both the King and Princess recover.
Asked about his well-being last week, she said her husband was “doing his best” as she opened a cancer support centre at the Royal Free Hospital in London.
Asked about King’s public schedule following the cancer diagnosis, a spokesman said: “Regrettably, a number of the King’s forthcoming public engagements will have to be rearranged or postponed.
“His Majesty would like to apologise to all those who may be disappointed or inconvenienced as a consequence.”
The Prince of Wales will return to public duty on Wednesday for an investiture and an evening fundraising gala for the London Air Ambulance.
The Duke of Sussex is flying back from California to see his father, while the Duchess and their two children will remain at home in Montecito.
A source close to the Sussexes said: “The Duke did speak with his father about his diagnosis. He will be travelling to the UK to see His Majesty in the coming days.”
Prince Harry was last seen with the Royal family at the King’s Coronation in May, but left without spending time with relatives.
