The King paid tribute to "the selfless doctors and nurses" who have supported him and his family through the "uncertainties and anxieties of illness" as he reflected on the royals' difficult year in his Christmas Day speech.
In the pre-recorded message, filmed by Sky the Fitzrovia Chapel in London, Charles, 76, said in part: "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.
"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 King and his daughter-in-law, the Princess of Wales, were both diagnosed with cancer within weeks of one another, with Kate having completed chemotherapy in September, while Charles is set to continue treatment into 2025.
The monarch also touched upon the the importance of community, saying: "All of us go through some form of suffering at some stage in our life, be it mental or physical. The degree to which we help one another – and draw support from each other, be we people of faith or of none – 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."
The speech was broadcast at 3pm on Christmas Day, hours after the King and Queen led the royals to St Mary Magdalene church, including the Prince and Princess of Wales, and their three children.
Charles was pictured attending church in Sandringham last Sunday, after hosting extended members of his family at Buckingham Palace for a pre-Christmas lunch last week.
The King's Christmas speech in full
"Earlier this year, as we commemorated the 80th Anniversary of D-Day, the Queen and I had the enormous privilege of meeting, once again, the remarkable veterans of that very special generation who gave of themselves so courageously, on behalf of us all.
"Listening to these once-young service men and women touched us deeply as they spoke of their comrades, drawn from across the Commonwealth, who never returned and who now rest peacefully where they made the ultimate sacrifice. Their example of service and selflessness continues to inspire, across the generations.
"During previous commemorations we were able to console ourselves with the thought that these tragic events seldom happen in the modern era.
"But, on this Christmas Day, we cannot help but think of those for whom the devastating effects of conflict – in the Middle East, in Central Europe, in Africa and elsewhere – pose a daily threat to so many people’s lives and livelihoods.
"We also think of the humanitarian organisations working tirelessly to bring vital relief. After all, the Gospels speak so vividly of conflict and teach the values with which we can overcome it.
"The example that Jesus gave us is timeless and universal. It is to enter the world of those who suffer, to make a difference to their lives and so bring hope where there is despair.
"As the famous Christmas Carol, 'Once In Royal David’s City' reminds us, 'Our Saviour holy' 'came down to Earth from Heaven', lived among ‘the poor and mean and lowly’ and transformed the lives of those he met, through God’s ‘redeeming love’.
"That is the heart of the Nativity Story and we can hear its beat in the belief of all the great faiths in the love and mercy of God in times of joy and of suffering, calling us to bring light where there is darkness.
"All of us go through some form of suffering at some stage in our life, be it mental or physical. The degree to which we help one another – and draw support from each other, be we people of faith or of none – 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.
"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.
"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.
"On our recent visit to the South Pacific to attend the Commonwealth summit, I was reminded constantly of the strength which institutions, as well as individuals, can draw from one another. And of how diversity of culture, ethnicity and faith provides strength, not weakness.
"Across the Commonwealth, we are held together by a willingness to listen to each other, to learn from one another and to find just how much we have in common. Because, through listening, we learn to respect our differences, to defeat prejudice, and to open up new possibilities.
"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.
"Again, listening is a recurrent theme of the Nativity story. Mary, the Mother of Jesus, listened to the Angel who revealed to her a different future full of hope for all people. The message of the Angels to the shepherds – that there should be peace on Earth – in fact echoes through all faiths and philosophies.
"It rings true to this day for people of goodwill across the world. And so it is with this in mind that I wish you, and all those you love, a most joyful and peaceful Christmas."
A history of the royal Christmas message
The King’s great-grandfather, King George V, delivered the first royal Christmas broadcast live on the radio from Sandringham 92 years ago.
He had reigned since 1910, but it was not until 1932 that he gave his first festive speech.
George V read a message composed by author Rudyard Kipling on the radio, known as the 'wireless'.
The original idea was suggested by Sir John Reith, the founding father of the BBC, to inaugurate the Empire Service, now the BBC World Service.
The fixed time of 3pm each year was chosen in 1932 because it was considered the best for reaching most of the countries in the British Empire by short wave.
George V’s eldest son, who became King Edward VIII in 1936, never delivered a Christmas speech as his reign lasted less than a year, ending in abdication.
King George VI, Edward's younger brother and Charles’ grandfather, made his first broadcast in December 1937 in which he thanked the nation and Empire for their support during the first year of his reign.
It was the outbreak of war in 1939 which firmly established the tradition, when George VI sought to reassure people and boost morale.
Following her father's death, Queen Elizabeth II made her first Christmas broadcast in 1952 and her first televised Christmas broadcast in 1957, live from Sandringham.
She delivered one every year except for 1969, when she decided the royals had been on TV enough that year following an unprecedented documentary the family made about their life.
In December 2022, Charles delivered the first Christmas broadcast of his reign.