Charles, 76, offered his "heartfelt thanks" to the doctors and nurses who helped him and the Princess of Wales during their cancer treatment.
Earlier, he and Queen Camilla, 77, had led their loved ones from Sandringham House to St Mary Magdalene Church.
They were followed by Princess Kate, 42, Prince William, also 42, and their children - Prince George, 11, Princess Charlotte, nine, and Prince Louis, six.
Kate wore a forest green coat and matching hat. Camilla's long green coat teamed with two sparkling brooches and a matching hat, was in a similar festive hue.
After church, the Royals stopped to chat with members of the public.
Kate, who completed her course of chemotherapy at the end of the summer, got a hug and was showered with flowers.
And in a touching moment, she expressed her gratitude to a well-wisher who had worked for Macmillan Cancer Support and who told Kate: "You are an inspiration to all the patients."
The Princess told the woman: "The amount of people who have written this year is extraordinary and I think cancer just really does resonate with so many families. People like you are doing all the hard work out there. I'm hugely grateful."
Another woman told Kate: "We're all behind you. Never forget that."
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