If there were an election tomorrow, and the main opposition party won a landslide victory, what would the duty of the current Prime Minister be?
No statute anywhere tells us. Certainly not the Fixed-term Parliaments Act, which says nothing about the issue.
The answer is however clear: he must resign. He must resign because he no longer has the confidence of the Commons and another person, in this example the leader of the opposition, is better placed to do so. This rule is found in Convention, not the posited law. It was this Convention that caused Gordon Brown to resign in 2010, before him John Major in 1997 and before that every other Prime Minister who lost the confidence of the Commons when another was better placed.
In some cases it may be unclear whether the Prime Minister has lost the confidence of the Commons, and so a vote is held to test the matter, as happened in December 1923. But what he or she must do after losing such a vote is not set down in any statute or other posited law anywhere.
The Fixed-term Parliaments Act
On the front-page of today’s Times Dr Catherine Haddon of the Institute for government is quoted as saying
In terms of a strict reading of the [Fixed-terms Parliaments Act] Boris is not required to resign. It is completely silent on all of this. The onus is on the incumbent Prime Minister – they get to choose whether they resign. If they do not it is hard for a new government to be formed without dragging the Queen into politics.
This statement is extremely misleading. It is true that the Fixed-term Parliaments Act is silent as to the Prime Minister’s duty to resign in favour of another better placed to command the confidence of the Commons. That is because it is legislation concerned with fixing Parliaments, not Governments (see its name). It is silent, just as is the Fisheries Act 1981 or the Contagious Disease Act 1864, because it concerns something else.
This blog began life four years ago to deal with this serious, and I thought dangerous, misreading of the Fixed-term Parliaments Act. It is a topic I have felt the need to return to.
The correct position is set out in the Cabinet Manual and in the House of Commons Public Administration and Constitutional Affairs Committee Report on the Act (see pp 18-25). A Prime Minister who does not have the confidence of the Commons when another is better placed must resign.
Conventions are unenforceable
If a Prime Minister were to defy Convention, and refuse to quit as the Times story suggests, and reports in the Telegraph claimed the Prime Minister’s advisor Mr Dominic Cummings has stated, what could be done?
Conventions are unenforceable. Indeed, courts cannot authoritatively rule upon them (save where they come up as an issue of fact as happened in the Spider Memos Case, and as questions of foreign law often do).
If a Prime Minister were so to disgrace his office by refusing to resign, destroying representative democracy in the United Kingdom, the only recourse middle aged conservative people such as myself personally have is to take to the streets. You will have to join me.
Confidence Motions
As the PACAC report on the Act makes clear, all that the Fixed-term Parliaments Act does in relation to a vote of no confidence is fix the form of it for purposes of starting the clock towards a General Election. It does not state what a vote of no confidence is, the form it must take, or what the Prime Minister should do if one is passed.
In order to leave the Prime Minister no option but to resign it must be clear that there is someone else better placed to command the confidence of the Commons. If there is a doubt about that the incumbent may say “ok, I don’t have the confidence of the majority, but nobody else does either, so I can continue, at least on a caretaker basis.”
If a vanilla no confidence motion in Johnson were passed (eg “this House has no confidence in Her Majesty’s Government”) the incumbent could, with some plausibility, claim that he did not have to resign if there were nobody else better placed. The Leader of the Opposition is not only unpopular with MPs from other parties but with many from his own, and it may be that there is a majority opposed to his being Prime Minister under any circumstances.
Stopping no deal Brexit
How could the legislature stop a determined Johnson government from pursuing a no deal Brexit?
The first method is legislation, a form of which I suggested here. Although some Conservative MPs may prefer this option it is difficult. First the government is in control of the legislature’s tabling of legislation. Second, even if a friendly Commons speaker were to defy Convention and allow legislation to be put, it would face stiff opposition in the Commons and Lords and it may be doubted whether it could be passed with sufficient speed.
The second and easier method is to pass a Confidence motion. This does not have to be in the form necessitated by the Fixed-term Parliaments Act. The government, by Convention, must make time for an opposition confidence motion. If it (completely outrageously) insisted that this could only be put forward in the form set out in the Act, the opposition could do so and it should be amended.
A motion of the form suggested below, clearly demonstrates the Commons’ lack of confidence in the Prime Minister, puts forward a person who does command the confidence of the House, and would not trigger the Fixed-term Parliaments Act as it is in a different form from the words used there
“This House, not wishing a General Election and wishing to stop a no deal-Brexit, has no confidence in her Majesty’s Government, and calls for the formation of a new Government led by the member for [X}.”
The person who then became Prime Minister could do so on a temporary basis for one purpose: seeking a further extension of UK membership of the European Union, in order for there to be a General Election or other vote. Who that person should be is of secondary importance (and indeed could be settled in the debate on the motion). MPs need however to start discussing names,
The Monarch
In our constitution today, the person of the Monarch is almost entirely symbolic. Although some powers are formally carried out through her, she is just a pen.
What if a motion of the above form were passed, and a Prime Minister so forgot himself so as to defy it, and stay in post? What should the monarch do?
In such extraordinary circumstances, she should dismiss him, and call on the person who commands the confidence of the House.
spinninghugo 
If both Conservatives and Labour are going to draw tribal red lines, perhaps its time to revisit the idea that “Lucas is out as from a party of one”?
Might this not be an advantage in this situation? The fact she has no established power base in the House means neither Labour nor Conservative MPs need fear an attempt to stay as PM at ‘their’ expense. The polls do not suggest they have much to fear in a general election. As for personal integrity its hard to imagine any MP who would be seen as more likely to stick to the limited mandate we are talking about.
From a political and real politik point of view if this is to work, it will have to be Me Corbyn. From the perspective of Lib Dem’s and the courageous Tories willing to defy their whip to prevent the U.K. from “no dealing” I understand your misgivings. The mantra of one thing worse than a no deal is a Mr Corbyn government has been repeated for so long many are hypnotised into forgetting all of the damage done so far by a Conservative government in the hash of these negotiations with the EU and its entire post-referendum strategy could never be excelled by anything in his remotest dreams an alleged Marxist could hope to do within the constraints of his mandate and Parliamentary sovereignty. The Official Opposition is just that. So if the Lib Dem’s and noble warriors like Dominic Grieve and Kenneth Clarke truly wish to register, as Democrats , their protest to the greatest possible extent at the democratic behaviour of the government and its incompetence, it would be to support the only possible way out of this, given all of the intricacies outlined above by supporting Mr Corbyn as Leader of the Opposition. The terms of the contract with him can be stated in the motion: extension and election and/or confirmatory referendum.