the queen becomes a republican

By this I mean she decides it is time for the monarchy to end. How could she do this? Could she leave instructions in her will? Abdicate and declare all her heirs to be invalid? Would she have to convince her prime minister to pass a bill? Would Charles and the rest of the succession need to agree?

According to the Brits on this borad, all she would have to do is refuse Royal Assent or go against her ministers’ “advise”.

Has she the power to not only abdicate, but stop all others from taking the throne?

The Monarch can’t unilaterally impose republicanism on a country. They don’t have that power. Sure, they can advocate for it, and they can quit their job as national mascot, but that just means the country chooses another national mascot.

It’s quite clear that in a modern European monarchy the monarch serves at the pleasure of the people. So if they don’t behave themselves according to the formal and informal norms that the people expect from their monarch, they can be replaced with someone who will. Or they could decide that after the previous example of the monarch behaving rudely that maybe they don’t need a monarch after all. Monarchs are not chosen by God any more.

IIRC, the question of who the monarch is and who the successor is set by Act of Parliament, so the Queen cannot unilaterally abdicate nor declare any of her heirs invalid. She’d need to get Parliament to pass a bill abolishing the monarchy and stipulating who the office of head of state is to devolve unto, to which she could then, in her final act as sovereign, give her assent.

Edward the 8th did for all his descendants.

“The Queen reigns but she does not rule.”

Her Majesty does not have any power to legislate unilaterally. Laws are made by Parliament, not by the Queen. Fundamental constitutional change of this sort can only be made by Parliament.

But he didn’t stop all others, so that would be a “no”, correct?

No. He indicated his intent to abdicate, and to renounce any rights for his descendants.

Parliament then passed the Act of Abdication to implement the King’s desire to abdicate. The Act also provided that none of his issue would have any claim to the throne. Edward gave Royal Assent to the Act, and that Act is what removed his putative descendants from the line of succession.

That Act was necessary to change the line of succession, as set out in the Act of Settlement of 1701. The King could not unilaterally change the line of succession, because the King does not have the power to legislate unilaterally.

But he needed a law passed by Parliament in order to do so.

No. The Act of Settlement is the law governing succession to the throne. Her Majesty has no power to legislate unilaterally and change the law as set out in the Act of Settlement.

She probably doesn’t even have the power to abdicate unilaterally, because the law says she is the monarch. An individual cannot change the law. An Act of Parliament is needed.

Side question: Has she the power to stop the next-in-line from taking the throne after she vacates it?

No, that would also require an act of Parliament.

Though she can give assent to a bill changing the order of succession, of course. In practice, if HM calls on Parliament to abolish the monarchy it’s unlikely that they would not agree, though working out who becomes the new head of state and what happens to Commonwealth states who HM is queen of and so on would take time.

Could parliament even debate such a bill in light of the treason felony act of 1848 or the treason act of 1702.

Of course. Parliamentary supremacy: parliament cannot bind its successors (or even itself).

Formally it would require notification via Message to the House from Her Majesty that She is content for the debate to go ahead.

As Edward VIII did, in granting Royal Assent to the Act of Abdication.

Why? why does one person get to decide the constitutional structure of the country and the Commonwealth? If HM says that she’s done being Queen and thinks the monarchy should be abolished, that’s for the people of each of her realms to decide, through Parliament, not for her.

That’s a bit of an understatement, at least for Canada. It would take unanimous consent of the federal government and all the provinces to shift Canada from a monarchy to a republic. That would take quite a bit of time, in my estimation. Her Majesty doesn’t have the power to dictate that to the Canadian people.

Perhaps but how do they pass a bill if until the bill is passed it is illegal to introduce the bill.

So if the parliament of UK dissolves the monarchy does Canada have some place where the queen could live?