The King has started treatment after being diagnosed with cancer - with the monarch stepping back from public duties on the advice of doctors.
Members of the Royal Family are thought to often keep their health problems a secret, but Buckingham Palace has said the King chose to "share his diagnosis to prevent speculation".
The statement adds that he also shared his diagnosis in the "hope it may assist public understanding for all those around the world who are affected by cancer".
Follow latest: King diagnosed with cancer
Here, we look at other cases of British monarchs who have been diagnosed with a serious health condition.
Queen Elizabeth II
The King's mother, Queen Elizabeth II, generally lived a life of robust health, but was diagnosed with COVID in February 2022.
In the months before her death aged 96 in September 2022, there were concerns about her mobility after she missed the State Opening of Parliament that year.
In October 2021, the Queen spent a night in the King Edward VII Hospital in central London after cancelling a visit to Northern Ireland - it was not clear why she was in hospital.
Buckingham Palace said she was admitted for "preliminary investigations" but returned to Windsor Castle a day later and "remained in good spirits".
It was her first overnight hospital stay in eight years - with the previous time being due to a case of gastroenteritis in 2013.
Prince Philip, the husband of Queen Elizabeth II and King Charles's father, died aged 99 in April 2021.
A month earlier he had a "successful procedure" for a pre-existing heart condition.
In 2012, he missed the Queen's Diamond Jubilee celebrations as he was treated in hospital for several days with a bladder infection.
In 2011, the Duke of Edinburgh was rushed to the hospital by helicopter from Sandringham due to chest pains.
He was treated for a blocked coronary artery and underwent a minimally invasive coronary stent procedure.
Two years earlier, Prince Philip was involved in a car crash when his vehicle was hit by another car.
He was not injured but was seen by a doctor as a precaution.
The Queen's sister had several strokes in the years before her death in 2002.
In 1999, she severely scalded her feet in a hot bath, impacting her ability to walk.
Queen Elizabeth II's mother, who died aged 101 in 2002, suffered persistent ulcers on her left leg and problems with her hips which made walking difficult in later life.
In 1995 she had her right hip replaced, before her left hip was replaced three years later.
King George VI
In 1951, King George VI, the father of Queen Elizabeth II, had part of his left lung removed.
He never made a full recovery and died a year later, when it was revealed he had been suffering from lung cancer.
King George VI was a heavy smoker and as a consequence had developed peripheral vascular disease causing intermittent claudication - meaning he experienced pain in his muscles due to a lack of oxygen.
Prince Edward VIII
Prince Edward VIII, who was King George VI's brother and Queen Elizabeth II's uncle, was King until he abdicated in January 1963.
Like his brother, he was a heavy smoker, and died in 1972 after suffering from throat cancer.
Sarah, Duchess of York
Sarah, Duchess of York, who was married to Prince Andrew, was diagnosed with skin cancer in January 2024 - just six months after being treated for breast cancer.
A spokesman for the duchess, 64, said she remained in "good spirits" despite the diagnosis.
Kate, Princess of Wales
In January, the Princess of Wales underwent abdominal surgery in London and was recovering at home after a stay in hospital.
She has since returned home to Windsor, with Kensington Palace saying she is "making good progress".