While Charles is expected to take a step back from public life while he is cared for, it is understood he will continue working behind the scenes – carrying on with his state business and official papers contained in his famous red boxes. Buckingham Palace is yet to confirm what type of cancer the 75-year-old monarch has.
He recently visited the London Clinic private hospital for an enlarged prostate, but the palace has said he does not have prostate cancer. A spokesperson said a "separate issue of concern was noted" during the visit and that subsequent tests "identified a form of cancer".
Tourists and locals gathered outside Buckingham Palace sent their best wishes to the King. Justin Haden, 52, from Canada, said the news was “sad” and he hoped the King would “make a recovery”.
Lisa Nash, 61, said it must be hard to go through cancer "in the public eye". She added: “I love the royal family. I think they do a lot for this country and they will continue to do so, they’ll just pull together and hopefully take some of the burden off of him and he can concentrate on getting better.”
How King Charles’s cancer diagnosis will affect his royal duties (The Independent)
Rishi Sunak opened Tuesday’s Cabinet meeting by paying tribute to the King and wishing him a "speedy recovery". In his first answer at Treasury Questions in the Commons, chancellor Jeremy Hunt said: “Mr Speaker, could I start by adding my comments to yours yesterday about His Majesty the King and wish him and his family well, as well as saluting his courage in being so open about his condition.”
While Sunak has said the King's cancer has been caught "early", the diagnosis still raises questions about the future of the working royal family, which Charles had previously vowed to slim down to a smaller group of core members.
Here, Yahoo News has created an interactive table showing the line of the succession of the throne, with details on each of the family members.
Prince William, the eldest son of Charles and the late Princess Diana, is first in line to the throne. Even while Charles still remains King, William, the Prince of Wales, is expected to take on more responsibilities as his father takes a break from public engagements.
The heir, 41, was due to return to official duties on Wednesday for the first time since his wife Kate spent two weeks in hospital after undergoing abdominal surgery. He'd postponed engagements to help look after the couple's three children, Prince George, 10, Princess Charlotte, eight, and Prince Louis, five, who are next in line to the throne in that order.
Prince Harry, 39, who reportedly landed back in the UK on Tuesday, is next in line. However, given his desire to start a new life with his wife Meghan Markle in California after stepping down as a working royal, it is unclear if he'd decide to take on the role in the unlikely event his turn came. Next in line is his son, Prince Archie of Sussex, aged four, and his two-year-old daughter Princess Lilibet of Sussex.
Prince Andrew, 63, comes next, and again, it's unlikely his turn would come within his lifetime. Even if it did, there's a good chance he wouldn't accept the job given his friendship with Jeffrey Epstein and the ensuing embarrassment for the royal family that came as a result.
Stepping in for the King
No date has been set for the King’s return to full public duties.
William, Queen Camilla and Anne, the Princess Royal, will be the key royals holding the fort, along with Edward and Sophie, the Duke and Duchess of Edinburgh. The Princess of Wales is expected to be out of action and away from public duties until after Easter.
Charles – a known workaholic – will carry on behind the scenes dealing with his famous red boxes – which contain his state business and official papers. He will also hold his weekly audience with the PM and monthly Privy Council meetings, which can both be done remotely.
The King and Queen were expected to visit Canada in May, and Australia, New Zealand and Samoa for the Commonwealth Heads of Government Meeting in October. However, Charles’s illness raises the prospect that William may have to deputise for the King, who is head of the Commonwealth, just like Charles did for the late Queen in the past.
What is a regency, and how would it work if the King's health worsened?
If Charles was incapacitated due to his illness, the Prince of Wales could be appointed Prince Regent through the Regency Act 1937, taking over the King’s duties.
As mentioned, it is unlikely to reach this stage as Charles's cancer has been caught early, but if it did, it would be the first time in more than 200 years that the British monarchy has required a regency, with the last Prince Regent being the future George IV.
A period of regency allows the King to transfer his powers as monarch to the Prince of Wales without having to abdicate. It means Charles would remain King and head of state, but William would be able to perform royal functions in the name and on behalf of Charles.
The Regency Act 1937 states that the monarch’s duties will be performed by a regent if the monarch is declared to be “by reason of infirmity of mind or body” incapable of performing royal functions, or if there is “evidence that the Sovereign is for some definite cause not available for the performance of those functions”.
This must be declared in writing by three or more out of the following: the sovereign’s wife the Queen, the Lord Chancellor, the Speaker of the House of Commons, the Lady Chief Justice and the Master of the Rolls.
There are a few reasons why Charles might not simply abdicate, particularly the abdication of the Queen's uncle Edward VIII in 1936, which sparked a major constitutional crisis and had a devastating impact on the royal family.