Wait, will Prince William be interim King?

King Charles III’s cancer diagnosis will turn minds to the question of what happens if he becomes unable to fulfil his constitutional duties. 

Buckingham Palace has announced he will continue performing his official paperwork and his weekly meetings with the prime minister throughout his treatment.

But what happens if he becomes seriously ill?

There are three options: counsellors of state, regency and abdication.

Counsellors of state

First, King Charles can delegate some or most of his royal functions to counsellors of state, as happens most commonly when he is travelling overseas. Two counsellors of state act jointly in exercising royal powers such as assenting to laws, receiving ambassadors and holding Privy Council meetings.

The counsellors of state are the spouse of the sovereign and the next four adults in line of succession to the throne – being Queen Camilla, Prince William, Prince Harry, Prince Andrew and Princess Beatrice.

However, Prince Harry is excluded while he is outside the United Kingdom, and in practice Prince Andrew and Princess Beatrice are not called on to act as they are not “working royals”.

As this left only Queen Camilla and Prince William to perform the role, a law was passed in the UK in 2022 to add Princess Anne and Prince Edward to the list.

Counsellors of state may carry out most of the sovereign’s functions while he is ill, but they cannot dissolve parliament, except on his instruction, and they cannot create peers. Whether they can appoint a prime minister remains a matter of debate. Most significantly, they cannot exercise powers with respect to the King’s other realms, such as Australia.