Rishi Sunak has wished the King a "speedy recovery" after Buckingham Palace announced he was diagnosed with a form of cancer.
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."
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. Best wishes to Charles III for a full and speedy recovery."
Ms Truss said: "He will be in our thoughts and prayers. God Save The King!".
Sir Tony said: "Wishing His Majesty the King a full, swift recovery and return to excellent good health."
The messages poured in from across the political spectrum, with Sir Keir Starmer wishing the King a full recovery "on behalf of the Labour Party".
"We look forward to seeing him back to swift full health," he added.
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."
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.
Health and Social Care Secretary Victoria Atkins wrote on X: "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."
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."