In February, the palace revealed the 76-year-old, who became king in 2022, had been diagnosed with an unspecified form of cancer detected in tests after a corrective procedure for an enlarged prostate.
While he was able to return to public duties two months later, the number of engagements has been limited on medical advice - something which the noted workaholic has found difficult.
"His treatment has been moving in a positive direction and as a managed condition the treatment cycle will continue into next year," the palace source said on Friday.
The source said there had been no change in the King's health and the news that his treatment would continue in 2025 did not represent any significant update.
But his busy pre-Christmas schedule, which concluded on Friday with a visit to the northeast London district of Walthamstow that staged a large counter-protest in August in response to nationwide rioting, was an indication of his determination to stay busy.
In October, Charles and his wife, Queen Camilla, made a brief stopover in India where they stayed at a holistic health centre following his first major trip since being diagnosed with cancer to Australia and Samoa.
Overall the last year has been difficult for the royals.
The disclosure in March that the King's daughter-in-law Catherine, the wife of heir Prince William, was undergoing preventative chemotherapy for cancer was another shock.
While the Princess of Wales' treatment has now ended, her return to official engagements has been limited and she said her path to full recovery would be long.
William said it had been the hardest 12 months of his life and "brutal" for the family.
But it has not just been health issues that have put the Windsors in the spotlight.
The King's younger brother Prince Andrew was embroiled in another scandal this month after a close business associate of his was banned from Britain over government suspicions he was a Chinese agent.
The royal finances have also come under media scrutiny while Charles was heckled by an Indigenous senator at Australia's Parliament House during his tour there, a reflection of ongoing questions about Britain's colonial past.
Meanwhile, Charles' younger son Prince Harry remains estranged from the family and more royal secrets are likely to be aired when he gives days of evidence in the witness box in his lawsuit against Rupert Murdoch's British newspaper group.
Both Harry and Andrew will be absent when the royals gather for their traditional festive gathering at the King's Sandringham home in eastern England, a very visual demonstration of those problems.
