Charles, 75, has postponed his public engagements and other royals will fill in for him.
But the Palace has stressed the King will continue in his role as head of state of the UK and the other Commonwealth Realms, including Australia.
The King’s cancer: What we know
While the Palace has not revealed what type of cancer Charles has, it’s been confirmed that it’s not prostate cancer - despite the cancer being discovered during his recent treatment for an enlarged prostate.
King Charles turned 75 in November last year. The Guardian reports that the most common cancers among men aged 75 and over in the UK are prostate, lung, bowel and bladder cancer, followed by melanoma skin cancer.
The Palace has also stayed vague about what Charles’ cancer treatment will look like, saying only that the King will now undergo “a schedule of regular treatments”. That could include chemotherapy, radiotherapy or drug treatments.
Harry coming home after phone call
It is understood the King personally told his sons princes William and Harry about the diagnosis as well as princes Edward and Andrew and Princess Anne.
“The Duke did speak with his father about his diagnosis. He will be travelling to UK to see His Majesty in the coming days,” a source told The Sun.
The outlet reports that Harry’s wife Meghan Markle is expected to stay at their Montecito, California home with their children, four-year-old Archie and two-year-old Lilibet.
On Sunday, Charles went to church with Queen Camilla in his first public outing since his hospital stay. The King travelled on Monday from his home in Sandringham, in Norfolk in England’s east, to London to begin his treatment as an outpatient.
The Sun has reported that Queen Camilla is preparing to play an important role in supporting her husband throughout his period of treatment while continuing with a full program of public duties.
King is ‘wholly positive’
In a statement, Buckingham Palace said that during the King’s recent hospital procedure for benign prostate enlargement, “a separate issue of concern was noted”.
“Subsequent diagnostic tests have identified a form of cancer.
“His Majesty has today commenced a schedule of regular treatments, during which time he has been advised by doctors to postpone public-facing duties.
“Throughout this period, His Majesty will continue to undertake State business and official paperwork as usual.”
As such there will be no need for “counsellors of state,” the monarch’s official stand ins, to take up their roles.
The Palace said Charles remained “wholly positive” about his treatment and said he “looks forward to returning to full public duty as soon as possible”.
Charles seen just one day before cancer announcement
The news of King Charles’ cancer diagnosis has come as a shock to many, as it came just one day after he was seen waving to crowds as he attended a church service in Sandringham.
With wife Camilla in tow, the King appeared in good spirits and on the mend from his recent prostate surgery.
PMs and presidents send best
Prime Minister Anthony Albanese has wished the 75-year-old monarch a “speedy recovery”.
“Today the thoughts of all Australians are with King Charles and his family,” Mr Albanese said in Canberra on Tuesday morning.
“We wish him very much a speedy recovery. I will be sending a message to the Palace this morning and we hope that King Charles has a speedy recovery and a return to his duties as soon as possible.”
British Prime Minister Rishi Sunak said all of the UK will be wishing the King well.
“Wishing His Majesty a full and speedy recovery,” Mr Sunak said.
“I have no doubt he’ll be back to full strength in no time and I know the whole country will be wishing him well.”
Canadian PM Justin Trudeau said he was thinking of the King while he underwent treatment.
Canada, like Australia, has Charles III as head of state.
I, like Canadians across the country and people around the world, am thinking of His Majesty King Charles III as he undergoes treatment for cancer. We’re sending him our very best wishes – and hoping for a fast and full recovery.