The monarch, 75, is believed to have notified his younger son about his shock health news before he announced it to the world via a Buckingham Palace statement.
On Monday evening, the palace confirmed that Charles has been diagnosed with a form of cancer and has begun a schedule of regular treatments. And while he has postponed public-facing duties for the foreseeable, he remains "wholly positive" about his treatment.
He will continue to undertake state business and official paperwork as usual.
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The statement read in part: "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.
King Charles latest
"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."
LISTEN: King Charles' health problems and Princess Kate's recovery
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