Here is a point that I feel should be borne out in this discussion. As a pure matter of law, King Charles is not “powerless”. A few reserve powers legally remain to him, and this does technically include the power to refuse assent to a bill passed by Parliament. It is true that the last time this power was used was in 1708 (when Queen Anne vetoed a Scottish militia bill – and even this last instance was upon the advice of her ministers). But I think that this explanation – that the Sovereign no longer uses this power “for fear of a constitutional crisis” (and of possibly becoming unpopular enough to lose the throne) is more accurate than to say that the King has “no power” or that he “must” sign every bill presented to him by Parliament. Legally, I believe that this simply is not true.
I will attempt to demonstrate this by means of a few simple arguments:
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The prerogative to give or withhold Royal Assent to a bill is a matter of common law rather than of written statute, but it has always been there. No bill becomes law before the Sovereign signifies Royal Assent to it. This is still true in 2025.
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Though fallen into disuse after 1708, no statute passed by the Westminster Parliament has ever taken away this power of the Sovereign, nor have the highest courts in the UK ruled that the power is now non-existent.
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There is no legal mechanism to force the King’s hand / to formally sanction him for refusing to signify his Royal Assent to a bill.
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The idea that the Sovereign acts on the advice of ministers / elected officials is a constitutional convention. The courts do not enforce constitutional conventions.
Therefore, I would posit that there is no “must” in it and that legally, the King can refuse his assent to any bill as things stand, and that bill will not become law at that moment. What long-term consequences it will lead to (the resignation of the government, the unpopularity of the King, the passage of a law stripping the King of his power of royal assent, the deposition of the monarchy and the installation of a republic, or whatever else might ensue) remains open to speculation, but in the moment, the king would be acting legally and no one could do anything to physically coerce or sanction him for doing so.
If anyone disagrees with my interpretation, please offer counterarguments.
I realize that the King actually doing so would probably result in a major shakeup of things (the situation with the Grand Duke of Luxembourg provides a possible outcome – the Luxembourg parliament stripped him of his veto power, which law I presume he signified his assent to. That said, prior to that, at the moment the Grand Duke refused to give his assent to the bill in question, he was acting perfectly legally and the bill didn’t get his assent, and was thus not a proper law at the time. Just like what I speculate above would happen if King Charles refused to assent to a British bill).
Compare this to the Emperor of Japan who, following the adoption of a more liberal constitution at the insistence of the Allies following World War II has officially no practical powers to speak of but is little more than a symbolic head of state. Or the King of Sweden – he also now has no powers at all enumerated in the Constitution. Not only is he not asked to give Royal Assent to bills, he is not even Commander-In-Chief of the military anymore. There’s some truly symbolic figureheads for you.
There is a notion that in the British monarchy (and other Commonwealth realms), the King or his viceroy (e.g. the Governor General and Lieutenant Governors in Canada) may quite legitimately exercise these few remaining disused powers if there is a crisis or exceptional situation, especially if their doing so would command popular support. There is the famous case in Australia of the 1975 dismissal of Prime Minister Gough Whitlam, for example, by the Governor General. I would highly recommend watching this video presented by Anne Twomey, Australian constitutional expert, on her channel Constitutional Clarion, analyzing the legitimacy of the whole affair. It might put some things into perspective.
In Canada, the Constitution Act, 1867 explicitly states that the Governor General may signify his assent to a bill passed by Parliament, refuse assent, or reserve the decision to the Sovereign. Moreover, the Sovereign has a period in which they can disallow a bill. The Lieutenant Governors of the provinces are also explicitly allowed to give or withhold assent to bills passed by the provincial legislature, or to reserve the decision to the Governor General. Now, it is my understanding that no Governor General has ever refused assent to a federal bill, but a number of provincial Lieutenant Governors have refused or reserved assent to provincial bills. In one case, assent was refused to a bill concerning banking that the LG thought was unconstitutional, but in another, IIRC, the LG refused assent to a bill ending prohibition in his province, which might have been more a matter of his own personal discretion. A veto or reservation of provincial legislation last happened in I think 1961, and would not be likely to happen today; all I’m saying is that in Canada there are more recent precedents for the exercise of the Royal Prerogative than in the UK.