The King has offered an emotional thank you to the 'selfless' medics who have supported his family this year.
In his annual Christmas Day broadcast yesterday, Charles candidly admitted to the 'uncertainties and anxieties of illness'.
The monarch, who has worked throughout his ongoing cancer treatment, gave an unprecedented glimpse into the depths of his personal battle, as well as that of his daughter-in-law, the Princess of Wales.
Charles, 76, has been praised for approaching his diagnosis and treatment with 'not a shred of self-pity', but he felt his speech should acknowledge the challenges of the year by paying tribute to those working in healthcare.
The King highlighted how much it has meant to him to hear words of encouragement from people who welcomed his decision to make his diagnosis public in the hope that it might help others.
He said: 'From a personal point of view, I offer special, heartfelt thanks to the selfless doctors and nurses who, this year, have supported me and other members of my family through the uncertainties and anxieties of illness, and have helped provide the strength, care and comfort we have needed.
'I am deeply grateful, too, to all those who have offered us their own kind words of sympathy and encouragement.'
The Christmas message was filmed for only the fourth time 'on location', with the King choosing a former hospital chapel in London to emphasise the link with the nation's vital healthcare community.
King Charles took the opportunity to thank nurses and doctors in his Christmas address
He also spoke of his 'deep sense of pride' at the responses to the riots over the summer
The King referenced his visit to a Commonwealth summit in Samoa in October and suggested the institution's strength lay in its 'diversity of culture, ethnicity and faith'
He said: 'I am speaking to you today from the chapel of the former Middlesex Hospital in London – now itself a vibrant community space – and thinking especially of the many thousands of professionals and volunteers here in the United Kingdom and across the Commonwealth who, with their skills and out of the goodness of their heart, care for others – often at some cost to themselves.'
His broadcast came just hours after more than 2,000 members of the public, some queuing overnight, gathered at Sandringham to greet the Royal Family for their annual walk to church on Christmas Day.
One former cancer nurse told Kate, 42, she was an inspiration to other patients after her moving video address last March in which she faced head-on vile gossip and speculation about her absence from public life.
In it, she confirmed she had also been diagnosed with the disease and forced to undergo a course of preventative chemotherapy following major abdominal surgery in January.
'People like you are doing all the hard work out here. I'm hugely grateful,' the princess replied, echoing her father-in-law's words.
Smiling and waving, it was clear that the King was buoyed by the presence of his family at the largest royal gathering for several years, which also included for the first time his stepchildren and grandchildren keen to offer their support after the challenges of the past 12 months.
Charles has not specified the form of cancer he was diagnosed with following benign prostate surgery, or the treatment he has been given, but aides have confirmed it will continue well into the new year to give him the best possible chance of beating the disease.
Keen to strike an upbeat tone to his message, His Majesty also spoke of his 'deep sense of pride' at communities across the UK who came together following rioting after the fatal stabbing of three little girls in Southport in July to show that unity is stronger than division.
Highlights of the royal year were shown as the King addressed the nation
Buckingham Palace is shown in a video of the highlights of the year for the King's address
Crowds gather to meet Queen Camilla in footage shared in the King's speech
Princess Anne is pictured in a collection of highlights from the year
King Charles and Queen Camilla arrived at Sandringham this morning
The King and Queen have had a difficult year, with his cancer diagnosis in February
Both Queen Camilla and Princess Catherine donned emerald green coats as they walked with King Charles and Prince William
Charles waved to onlookers as Catherine held hands with her Prince Louis, her youngest child
Princess Catherine strolled with Prince William and their three children in a coat designed by Sarah Burton for Alexander McQueen
The first people in a queue of well-wishers waiting to see members of the royal family walk to church arrived on Tuesday and camped overnight
Pomeranians Peanut and Coco ahead of the Christmas Day morning church service
He said: 'I felt a deep sense of pride here in the United Kingdom when, in response to anger and lawlessness in several towns this summer, communities came together, not to repeat these behaviours but to repair.
'To repair not just buildings, but relationships. And, most importantly, to repair trust; by listening and, through understanding, deciding how to act for the good of all.'
Saying that 'all of us go through some form of suffering at some stage in our life, be it mental or physical', he emphasised how it is the degree to which we reach out to help one another, regardless of nationality or faith, that is a 'measure of our civilisation as nations'.
'This is what continually impresses me, as my family and I meet with, and listen to, those who dedicate their lives to helping others,' he added.
There was also a clear reference to current conflicts in the Middle East and Ukraine, as well as ongoing civil wars across Africa, as he recalled the lessons of the past so memorably recalled at this year's 80th anniversary commemorations of D-Day.
And he highlighted the importance of the Commonwealth, of which he is now head, and its ability to 'respect our difference, defeat prejudice and open up new possibilities'.
Accompanying the broadcast was footage of Royal Family members hard at work over the year, as well as scenes of his grandchildren, Prince George, Princess Charlotte and Prince Louis, at Trooping the Colour.
The three young royals stole the show, as always, at his side at Sandringham yesterday – Louis in particular looking delighted as he was encouraged by his mother to collect armfuls of toys, chocolate and gifts.
Notably absent, not for the first time, was Prince Harry – who remains estranged from his father, brother, and the wider Royal Family – as well as the Duke of York.
The latter is also a non-working royal but has previously been permitted to join the family in Norfolk. However just a week ago he was forced to pull out after being involved in yet another public scandal over his relationship with a suspected Chinese spy.
