ONE of the Queen’s last official acts was to appoint Liz Truss as Prime Minister.

Appointing a prime ­minister is perhaps the monarch’s most ­important function. Like other royal functions, it must normally be performed at the behest of others, with little or no room for ­personal discretion.

In making the appointment, the monarch does not signal their personal approval for the Prime Minister or for the policy of the ­Government. The appointment is merely a recognition, ­emerging from the conventional rules of the so-called “British Constitution”, that the ­person to be appointed has a “right” to be appointed prime ­minister.

When Clement Atlee went to see George VI in 1945, he simply announced that he had won the election – and that was that. The King could not have objected.

Yet it is not always as simple as all that. The conventional rules are not formally written down in any one binding authoritative source. The Cabinet Manual, which is quite terse on the subject, is as close as it gets.

Two different principles are in play.

The first is the traditional principle of ­parliamentary government: the monarch ­appoints as prime minister the member of the House of Commons who “best commands the confidence” of the House. The Prime Minister remains in office by virtue of parliamentary ­confidence.

The second is the principle of party ­government: the monarch appoints as prime minister the “leader of the majority party in the House of Commons” and that the Prime ­Minister remains in office by virtue of the ­confidence of their party.

Normally, these two principles are in ­harmony and no practical difficultly arises. However, we do not live in normal times. As the UK unravels, party politics have become more polarised, more fractious, more unstable, more unpredictable.

It was clear from the earlier rounds of voting that Liz Truss did not have majority support amongst Conservative MPs. Hypothetically, if relying on the “best commands the confidence of the House” principle, the Queen could have asked Rishi Sunak, as the front-runner amongst Tory MPs, to form a government.

That the Queen did not even for a moment consider doing so indicates that it is now formal election to the party leadership, not ­parliamentary support, that really counts.

Part of the difficulty arises from the fact that party leaders are now chosen by the party’s members, rather than by its MPs. This change, brought about by reforms to the internal rules of political parties, has the potential to throw the old unwritten constitution into disarray.

For one thing, it contributes to political ­instability. Party discipline and the instinct for self-preservation will cover a multitude of ­misgivings – at least for a while – but a prime minister who does not have the support of his or her parliamentary party is already in a ­precarious position.

It also tends to lower the quality of prime ministers. MPs know the characters of potential prime ministers, they have seen them perform up-close. Party members in the country at large have to rely on the media spin.

Above all, it skews the political spectrum to the extremes. Truss was chosen by a small ­selectorate of Tory members – a tiny, ­unrepresentative slice of the population, skewed heavily towards the old, the southern English, and the propertied. Since party members tend to be more ideologically extreme either than MPs or voters, a leader chosen by the ­membership has more incentive to pursue agendas that are popular with their party base, no matter how unpalatable to the general public.

There are solutions to this. One is to require party leaders to be chosen (at least when in ­government), by the party’s parliamentarians, and not by the national membership. That could written into a future ­constitution.

A second potential solution, at least ­worthy of discussion, is a constitutional ­requirement that a general election be held within 12 months ­following a change of the leader of the ­governing party. That would discourage MPs from ­ousting a prime minister too lightly, while also ­disciplining their choice of successor.

Of course, there is a limit to what can be achieved by such tinkering. There are ­other ­issues, including electoral reform, party ­funding, the role of lobbyists, and media ­integrity, to be considered.

Beyond that, more fundamentally, there are serious concerns about the health of the whole unwritten constitutional order. Its days are numbered. Eras end. All things pass away. Old certainties come to an end.

The accession of Charles III provides an ­opportunity to re-evaluate the constitutional ­situation and to consider whether “making do and muddling through”, on the basis of the Hanoverian settlement and Victorian conventions, still meets the needs of Scotland in the 21st century.

We would be better served by a modern ­democratic constitution in an independent, post-Union state.