King Charles has been diagnosed with a form of cancer and has begun treatment, Buckingham Palace has said.
The shock announcement comes a week after the King, 75, was discharged from a private London hospital after a scheduled treatment for an enlarged prostate.
A statement released by the Palace at 6pm on Monday said: "During the King’s recent hospital procedure for benign prostate enlargement, a separate issue of concern was noted. Subsequent diagnostic tests have identified a form of cancer.
"His Majesty has today commenced a schedule of regular treatments, during which time he has been advised by doctors to postpone public-facing duties. Throughout this period, His Majesty will continue to undertake State business and official paperwork as usual.
"The King is grateful to his medical team for their swift intervention, which was made possible thanks to his recent hospital procedure. He remains wholly positive about his treatment and looks forward to returning to full public duty as soon as possible.
"His Majesty has chosen to share his diagnosis to prevent speculation and in the hope it may assist public understanding for all those around the world who are affected by cancer."
The King spent three nights undergoing treatment for an enlarged prostate at the London Clinic before being discharged last Monday.
He had gone to Sandringham to continue his recovery. He was seen on Sunday attending a church service in his first public outing since his hospital stay, waving to the cameras as he attended the service with Queen Camilla.
But the King, 75, returned to London on Monday morning and the Palace says he has commenced treatment as an outpatient.
Details of his diagnosis have not been revealed by the Palace, but they have confirmed Charles does not have prostate cancer.
Charles also told Prince William personally, along with this three siblings Princess Anne, Prince Andrew and Prince Edward.
The King has rearranged or postponed any forthcoming public engagements.
A Palace spokesman added: "His Majesty would like to apologise to all those who may be disappointed or inconvenienced as a consequence."
It is understood to be too early to say when Charles will return to full public duties, although he is said to be looking forward to doing so as soon as possible.
He will continue with weekly audiences with the Prime Minister Rishi Sunak.
Mr Sunak, writing on X, said: “Wishing His Majesty a full and speedy recovery.
“I have no doubt he’ll be back to full strength in no time and I know the whole country will be wishing him well.”
Labour leader Sir Keir Starmer wrote on X: "On behalf of the Labour Party, I wish His Majesty all the very best for his recovery. We look forward to seeing him back to swift full health."
Northern Ireland’s First Minister Michelle O’Neill wished the King a full and speedy recovery, along with Mark Drakeford, the First Minister of Wales, and Scottish First Minister Humza Yousaf.
While messages of support for the King flooded in from around the world, with US president Joe Biden among the well wishers, speaking of his" concern".
Health professionals have thanked the King for raising awareness about cancer amid praise of his "openness and honesty" in sharing his personal health news.
Dr Jay Verma, president of the GP section at the Royal Society of Medicine, thanked Charles for using his "unique position" to highlight how cancer can affect anyone.
He said: "We wish His Majesty a speedy recovery and success in his treatment for cancer.
"We have made leaps and bounds in our cancer treatment and survival rates, and the earlier a problem is diagnosed, the more options there are for treatment."
Although he will pause his public events, the King will continue with his constitutional role as head of state, including paperwork and private meetings.
There is a constitutional mechanism for when the head of state is unable to carry out official duties - in that circumstance "counsellors of state" can be appointed to stand in for the monarch.
At present that includes Queen Camilla, Prince William, Princess Anne and Prince Edward, with Prince Harry and Prince Andrew no longer to be called on as non-working royals.
Prince William had also temporarily withdrawn from public engagements while he helped his wife Catherine, the Princess of Wales, as she recovered from "abdominal surgery".