He was diagnosed after a “separate issue of concern was noted” and was investigated while he was being treated for a benign enlarged prostate.
The last time the 75-year-old King was seen was on Tuesday, as he left Clarence House the day after starting his treatment, following a brief reunion with the Duke of Sussex.
On Saturday, the King spoke about his diagnosis for the first time as he thanked people for their “many messages of support and good wishes”. He added in his statement that it was “equally heartening” to hear how sharing his diagnosis has helped to promote public understanding of cancer.
Charles has postponed all public-facing duties, but is continuing with behind-the-scenes work on his red boxes of state papers.
Since his diagnosis, the family has rallied around Charles. Other members of the royal family are stepping up to take on additional duties and public engagements while the monarch undergoes treatment.
The Princess Royal was the first to return to royal duties on Tuesday when she carried out an investiture ceremony at Windsor Castle before heading to Nottingham on an official visit and then to London to make a speech in the evening.
The Queen has rarely left the King’s side during his recent health scare, but she has also resumed her duties this week, where she gave a brief update on her husband’s health.
Prince William also returned to royal duties this week after he took a break to focus on his family and wife, the Princess of Wales, who is recovering from her abdominal surgery.
The future King did not, however, see his brother, the Duke of Sussex, on his brief visit to the UK.
Prince Harry put his rift with his father aside and cleared his schedule to fly from California to see the King on Tuesday.
After flying over 5,000 miles, Harry spent around 30 minutes with his father at Clarence House in London before staying in a hotel and returning back to Los Angeles just 26 hours after he had arrived.
Meanwhile, the King has been staying at Sandringham since he underwent his first round of cancer treatment. One of the most famous stately homes in Britain, Sandringham sits on an 8,000-hectare estate in Norfolk on the eastern coast of England.
The Palace has called for the King’s privacy to be respected, especially during his treatment, but said he wanted to make his diagnosis public because of his long-running support for cancer charities.
A host of medical professionals from different groups have commended the King’s candour over his health.
Chiara De Biase, director of support and influencing at Prostate Cancer UK, highlighted a 500 per cent increase in people coming to the charity’s website for information and support in the week following the King’s announcement of his enlarged prostate condition in January.