Rishi Sunak has wished the King a “speedy recovery” after Buckingham Palace announced he was diagnosed with a form of cancer.
The prime minister led global well-wishes to the King, posting on X: “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.”
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The King has begun a schedule of regular treatments and has been advised to postpone his public-facing duties.
Former prime ministers Liz Truss, Boris Johnson and Sir Tony Blair posted similar messages, with Mr Johnson saying: “The whole country will be rooting for the King today.”
Across the Atlantic, Joe Biden said he was “concerned” about Charles and planned to call him later.
The US president told reporters: “I’m concerned about him. Just heard about his diagnosis.
“I’ll be talking to him, God willing.”
Donald Trump, Mr Biden’s predecessor, called the King a “wonderful man, who I got to know well during my presidency”.
Writing on his Truth Social platform in all capital letters, he added: “We all pray that he has a fast and full recovery!”
Canadian Prime Minister Justin Trudeau said: “I like Canadians across the country and people around the world, I am thinking of His Majesty King Charles III as he undergoes treatment for cancer.
“We’re sending him our very best wishes – and hoping for a fast and full recovery.”
In the UK, messages poured in from across the political spectrum, with leaders of all the main parties wishing the King a full recovery.
Labour leader Sir Keir Starmer tweeted: “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.”
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Full statement from Buckingham Palace
Lib Dem leader Sir Ed Davey said his party joins “the rest of the nation in wishing a full and quick recovery to His Majesty”.
Northern Ireland’s new First Minister, Michelle O’Neill, the first nationalist to occupy the office, said: “I am very sorry to hear of King Charles’ illness and I want to wish him well for his treatment, and a full and speedy recovery.”
The King was also praised for making his diagnosis public, with some MPs reflecting on their own experiences with cancer.
Defence Secretary Grant Shapps, who was diagnosed with Hodgkin’s lymphoma in his thirties, said: “As a cancer survivor, I know how impactful his decision to share this news will be in lifting understanding for those affected.”
Shadow health secretary Wes Streeting, who received treatment for kidney cancer in 2021, said: “One in two of us will develop cancer during our lives, but millions more are affected when someone they love is diagnosed with cancer.
“Sending best wishes to His Majesty for his treatment and to his family as they support him throughout.”
Home Secretary James Cleverly, whose wife has battled breast cancer, said: “Susie and I have seen the amazing work that medical professionals can do in cancer treatment. I wish His Majesty a full and speedy recovery.”
Chancellor Jeremy Hunt, who lost his brother to cancer last year, said: “Thinking about His Majesty the King and his family tonight. Cancer is a horrible disease but we all pray it has been caught early enough for him to continue his remarkable life of service to our country.”
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Royal commentator Alastair Bruce on diagnosis
Commons Speaker Sir Lindsay Hoyle told MPs: “I know the whole House will wish to join me in expressing our sympathies with His Majesty the King following the news announcement this evening.
“Our thoughts are, of course, with His Majesty and his family, and we’d all wish to send him our very best wishes for the successful treatment and a speedy recovery following tonight’s news.”
The diagnosis comes following treatment for an enlarged prostate, although the palace said he does not have prostate cancer.
The palace has not confirmed what type of cancer the 75-year-old monarch has.
The Duke of Sussex has spoken to his father about his diagnosis and will travel to Britain to see him in the coming days, said a source close to Prince Harry.
A palace spokesperson said: “His Majesty has been treated for benign prostate enlargement.
“It was during this intervention that the separate issue of concern was noted and subsequently diagnosed as a form of cancer.
“This second condition will now receive appropriate treatment.”