King Charles has been diagnosed with cancer, Buckingham Palace announced on Tuesday morning (NZ time).
The palace released a statement saying during the King's recent time in hospital to undergo a corrective procedure for an enlarged prostate, a "separate issue of concern" was found.
"Subsequent diagnostic tests have identified a form of cancer," the statement said.
"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 statement doesn't say what kind of cancer was found but Reuters reported a royal source confirmed it wasn't prostate cancer.
World leaders including Prime Minister Christopher Luxon have responded to the news.
"Kia Kaha King Charles. On behalf of all Kiwis I wish His Majesty all the very best for a speedy recovery," Luxon tweeted.
US President Joe Biden said: "I am concerned about him" and that he planned to call him later.
According to ITV Royal Editor Chris Ship, a Buckingham Palace spokesperson said the King returned from Sandringham to London on Monday morning (UK time) to commence treatment as an out-patient.
"He remains at home in London this evening."
Ship said the King is being treated as an outpatient so his constitutional role as Head of State is unaffected.
"He can still have meetings in person and by video call. And no plans to appoint the Counsellors of State - who can stand in for a Monarch," Ship wrote on X.
Ship added Prince William and likely all of King Charles' family were told in person by the Monarch.
Ship said a spokesperson for the Prince of Wales said he is in regular contact with his father.
The BBC reported Prince Harry has spoken to the King about his cancer diagnosis and will travel to the UK to see him in the coming days.
Harry now lives with his wife Meghan and their two children, Archie and Lilibet, in California after the couple stepped down from royal duties in 2020.
According to the Buckingham Palace statement, the King is "grateful" to his medical team for their swift intervention.
"He remains wholly positive about his treatment and looks forward to returning to full public duty as soon as possible," the statement said.
"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 was last seen in public at a church service in Sandringham on Britain's east coast on Sunday (UK time), where he waved to crowds.
UK politicians were quick to offer their support to the King, with Prime Minister Rishi Sunak wishing him 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," Sunak said on X.
Labour leader Sir Keir Starmer and Commons Speaker Sir Lyndsay Hoyle also wished the King a quick recovery.
The news comes days after Charles and his daughter-in-law left the same hospital where they had both undergone planned treatments.
Kate, who is the Princess of Wales and wife to Prince William who is the heir to the throne, spent two weeks in the London Clinic for abdominal surgery.
It was announced earlier on Tuesday morning (NZ time) that Prince William would return to office duties for the first time since Kate spent two weeks in hospital.
Charles, 75, became King in September 2022 following the death of his mother Queen Elizabeth.
