Sir Keir Starmer said he is "glad" that King Charles's cancer treatment will be scaled back in the New Year.
The King shared the encouraging news in a video message broadcast on Channel 4 as part of a special Stand Up To Cancer UK broadcast.
Taking to social media, the Prime Minister wrote: "A powerful message from His Majesty The King.
"I know I speak for the entire country when I say how glad I am that his cancer treatment will be reduced in the new year. Early cancer screening saves lives."
The Prime Minister said he is 'glad' that the King's cancer treatement will be scaled back | PA / CHANNEL 4 / PAThe King said it "troubles me deeply" that millions of people across the UK miss the opportunity for early cancer diagnosis.
The monarch urged the public to take up routine screening in a heartfelt plea.
His majesty asked viewers during the festive season to hold in their "hearts, and your minds and prayers" the hundreds of thousands of people diagnosed with cancer each year, as well as "the millions more who love and care for them".
Drawing on his own experience, the King said: "I know from my own experience that a cancer diagnosis can feel overwhelming.
Sir Keir described the King's address as a 'powerful message'
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"Yet I also know that early detection is the key that can transform treatment journeys, giving invaluable time to medical teams - and, to their patients, the precious gift of hope."
He told viewers that those gifts were ones "we can all help deliver", before praising what he described as the "community of care" surrounding cancer patients.
The King said he had been "profoundly moved" by the specialists, nurses, researchers and volunteers who "work tirelessly to save and improve lives".
However, he said he had learned something during his own cancer journey that caused him deep concern.
The King urged people to get scanned for cancer early
| CHANNEL 4 / BANGO STUDIOSThe monarch explained: "I have also learned something that troubles me deeply - at least nine million people in our country are not up to date with the cancer screenings available to them.
"That is at least nine million opportunities for early diagnosis being missed."
The King said the consequences of delayed diagnosis were clear, adding: "The statistics speak with stark clarity."
Citing bowel cancer as an example, he said: "When bowel cancer is caught at the earliest stage, around nine in 10 people survive for at least five years. When diagnosed late, that falls to just one in 10. Early diagnosis quite simply saves lives."
King Charles reflected on how discovering his illness at an early stage had allowed him to continue leading what he described as 'a full and active life' whilst undergoing treatment
| GETTYHe said he had heard the same message repeatedly during visits to cancer centres across the country and shared that early detection had made a significant difference in his own case.
"I know, too, what a difference it has made in my own case, enabling me to continue leading a full and active life, even while undergoing treatment," he said.
In a personal update, the King told viewers: "Today I am able to share with you the good news that thanks to early diagnosis, effective intervention and adherence to 'doctors' orders', my own schedule of cancer treatment can be reduced in the New Year."
He described this as "a personal blessing" and said he hoped it would offer encouragement to "the 50 per cent of us who will be diagnosed with the illness at some point in our lives".
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