American President Joe Biden said he was concerned about the King's cancer diagnosis and planned to call him, as messages of support for the King came in from around the world.
"I am concerned about him," Biden said when asked about the news during a visit to Las Vegas.
The President told reporters he had just heard about the King's diagnosis and added "I'll be talking to him, God willing".
Mr Biden later tweeted: "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."
Well wishes for the King have poured in from across the globe after Buckingham Palace announced he was diagnosed with a form of cancer.
Canadian Prime Minister Justin Trudeau said: "I, like Canadians across the country and people around the world, 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."
Former President Donald Trump wrote on his Truth Social network that the king was "a wonderful man" and that "we all pray that he has a fast and full recovery".
The President of Ireland Michael D Higgins also extended his best wishes to the King and his family.
Mr Higgins said he was "very sorry and concerned" about the King's diagnosis.
A statement on his behalf said: "The president has sent his best wishes on behalf of the people of Ireland to King Charles for his medical treatment and for his full recovery, as well as to Queen Camilla and his family."
Australian Prime Minister Anthony Albanese added: “The thoughts of all Australians are with King Charles and his family.
“We wish him very much a speedy recovery. I will be sending a message to the palace this morning and we hope that King Charles has a speedy recovery and a return to his duties as soon as possible."Charles has begun a schedule of regular treatments and has been advised to postpone his public-facing duties.
Messages for the King have been sent from across the political spectrum after the cancer diagnosis.
Prime Minister Rishi Sunak said 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.”
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. https://t.co/W4qe806gmv
— Rishi Sunak (@RishiSunak) February 5, 2024
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.”
Liberal Democrat leader Ed Davey posted: “The Liberal Democrats join the rest of the nation in wishing a full and quick recovery to His Majesty.”
London Mayor Sadiq Khan added: "On behalf of Londoners I wish His Majesty a speedy and full recovery. I look forward to his return to full health as soon as possible."
The King was also praised for making his diagnosis public.
His decision to share his diagnosis to assist public understanding for all those affected by cancer is commendable
Victoria Atkins, Health Secretary
Health and Social Care Secretary Victoria Atkins wrote on X: “My thoughts are with King Charles and the whole Royal Family.
“His decision to share his diagnosis to assist public understanding for all those affected by cancer is commendable.
“Wishing His Majesty the very best and look forward to seeing him resume his public duties.”
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.”
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.”
Mr Sunak’s immediate predecessors in No 10, Boris Johnson and Liz Truss, also sent their best wishes to the King, with Mr Johnson saying: “The whole country will be rooting for the King today.”
Welsh First Minister Mark Drakeford, Scottish First Minister Humza Yousaf, Northern Ireland’s First Minister Michelle O’Neill and deputy First Minister Emma Little-Pengelly also sent get-well wishes.
The Palace said the disease was found while the King was being treated in hospital for an enlarged prostate, but that it is not prostate cancer.
Charities supporting people with cancer have also sent the King their best wishes.
A statement from Macmillan Cancer Support on X said: "Our thoughts are with His Majesty the King and his family.
"We are sending our best wishes at what we know must be an incredibly challenging time."
The King has been a long-standing supporter of Macmillan, becoming patron of the charity in 1997.
Professor Pat Price, founder of the Catch Up With Cancer campaign, said: "In this worrying time, our hearts go out to the King and his family.
"The outpouring of well-wishes from millions of people reflects the collective concern we all share.
"The King's openness about his battle with cancer is a powerful reminder that one in two of us may face cancer at some point in our lives."
Judi Rhys, chief executive of Tenovus Cancer Care, of which the Princess Royal is patron, added: "We are saddened to hear of the King's cancer diagnosis and wish him all the best with his treatment and recovery.
"Our thoughts are with the whole of the royal family including our patron HRH the Princess Royal."
A statement from Cancer Research UK on X said: "We're sorry to hear the news that King Charles has been diagnosed with cancer.
"We're sending our thoughts and well wishes to the King and the royal family at this difficult time."