According to prime minister Rishi Sunak, the cancer was “caught early”. The two will continue meeting on a weekly basis, even as the monarch reduces his public appearances while he undergoes treatment.
The Palace has clarified that Charles will be back to his full-time role “as soon as possible”, although the Royal Family does have a system in place should the King feel the need to step back completely following his diagnosis.
Here’s how that would work.
Who would take over the decision-making?
Since the Regency Act of 1937, a Counsellor of State has always taken over constitutional duties during short-term absences from the monarch – that can include any kind illness or absence abroad.
They will carry out duties such as holding the Privy Council meetings, signing routine documents and receiving the credentials of new ambassadors to the UK.
This rule was created shortly after King George VI ascended the throne in 1936, following a period of turbulence where his older brother Edward VIII abdicated before his coronation so he could marry divorcee Wallis Simpson.
George’s heir to the throne was his daughter Princess Elizabeth, later Queen Elizabeth II, but she was just 10 at the time, so the new law was introduced in the event the monarch was out of action.
However, there are a few jobs the counsellors are not allowed to do while taking on the monarch’s duties.
That includes any matters related to the Commonwealth, the dissolution of parliament (unless acting on the monarch’s instruction), the creation of peers and appointing a PM.
There are five people who decide if the monarch is incapable of carrying out their duties; the monarch’s spouse, the Lord Chancellor, the House of Commons speaker, the Lady Chief Justice and Master of Rolls.
The monarch does not need to agree to their decision, and neither does the heir apparent.
What about a regency?
Letting a counsellor of state take over is very different to a regency.
That’s when someone else effectively holds all of the throne’s constitutional power because the monarch is indisposed for the long-term.
A regency would mean the next in line to the throne, Prince William, would take over completely.
Nine separate Regency Acts had to be drafted written into law to cover a whole range occasions between 1728 and the creation of the 1937 rule.
Who are the Counsellors of State?
Two or more Counsellors are appointed by Letters Patent to act in the King’s place.
These are the four adults next in the line of succession over the age of 21 and the monarch’s spouse.
The official royal website says Prince William (first in line), Prince Harry (fifth in line), the Duke of York (eight in line) and Princess Beatrice (ninth in line) are all counsellor of state, as is the King’s wife, Queen Camilla.
However, it is widely believed that neither Prince Harry nor Prince Andrew would take on the role, because they are not working royals anymore. Prince Edward and Princess Anne would therefore operate as replacement counsellors.
What happens now?
For now, Charles will continue his constitutional duties behind closed doors and cancel all public engagements.
That doesn’t mean working life is completely on hold, though. He will continue to grant royal assent to laws from the UK, Scottish and Welsh parliaments as well as the Northern Ireland assembly.
He will also approve some public appointments and all ministerial appointments, while still meeting Sunak on a weekly basis.
Buckingham Palace explained: “Regrettably, a number of the King’s forthcoming public engagements will have to be rearranged or postponed.
“His Majesty would like to apologise to all those who may be disappointed or inconvenienced as a consequence.”.
The Palace added that Charles “looks forward to returning to full public duty as soon as possible”.
Prince William is expected to take up his public duties again soon, once his wife, Kate, has recovered from her recent operation, but the pressure will be on now the King is ill, too – it means he is now the most senior royal in the public eye.
Queen Camilla will still be carrying out her own solo public engagements, while Prince Harry is flying from the US alone to visit his father in the coming days.