Why should the Queen endorse the unions’ decision to choose a new Labour PM mid-term?

Imagine that the prime minister of the day — whoever he might be — were to stand down as PM and leader of the majority party in Parliament. His party would choose a new leader. The new leader would presumably become prime minister. I say ‘presumably’ because, according to my understanding of our unwritten constitution, it would be for the monarch to decide that this new leader was the person best placed to command a working majority in the House of Commons and, having so decided, invite him to form a government. The monarch’s task (runs the argument) would simply be to rubber-stamp.

Or would it? I’ll hold to that ‘presumably’ while I put to you a hypothetical question — a silly enough hypothetical question, I grant, but designed to test a principle. Suppose a party was returned to government at a general election, led by a man who promised to serve for the full term of that new government. Suppose this party had adopted an internal constitution which provided that if a leader did stand down, his replacement was henceforward to be chosen not by the party’s MPs, but by — say — the general secretary of the Socialist International.

And suppose the prime minister broke his promise to stay, and stood down some years before any general election was expected. In which case (so long as the party’s MPs were content to have a leader imposed upon them) the monarch would have to empower the general secretary of the Socialist International to choose the next prime minister, without any intervening general election.

Of course the monarch could trigger such elections, by dissolving Parliament. I suggest that in these circumstances there would be an argument (though contested) that the monarch should decline to invite the new party leader to form a government before putting the arrangement to the popular plebiscite of a general election.