We’re back into the season of what the Australians call “spills” — potentially getting rid of a political party leader. In this case, the prime minister.

When these seasons came round almost dizzyingly fast in the May-Johnson-Truss-Sunak era, one sensible, important but boring thing achieved near consensus among political pundits. It does not make sense, and is politically dangerous, to have the prime minister directly elected by the paid-up members of one party.

If Sir Keir Starmer goes, before or after the May elections, we are bound to see it happen again, however bad it is. But a better system should be adopted voluntarily by all parties that have a realistic prospect of their leader becoming prime minister. (The same arises at Holyrood in respect of a first minister at Holyrood Palace — or Cardiff Bay — but they have one useful safeguard that is absent at Westminster).

Why is the present system so flawed?

Having party members elect leaders makes plenty of sense in opposition. It engages the membership, ensures the parties don’t get too far from their grass roots and gives parties a democratic basis. It is also constitutionally respectable. The party will go into an election with that leader as its candidate for No 10 and the electorate will be able to decide whether they want him or her. If the party chooses someone who doesn’t appeal to the electorate, it probably won’t get elected into Government.

But different factors apply when a sitting prime minister has to be replaced. At that point, it is not just a party leader being selected, but a new head of government. The rest of us, who aren’t members of that party, will be affected. It is not fair if neither we nor our representatives have any say.

Beyond being unfair, it can also weaken our constitution and make the workings of politics less smooth. In any organisation that needs to work well, the hiring should be aligned with accountability. If we elect a party at a General Election, we have all had some hand in putting the prime minister in Downing Street, give or take the distortions of the voting system. We have also elected the House of Commons, which then holds him or her to account.

But if the members of one party have effectively elected the prime minister, what happens? First, he or she may not be much liked by his or her own parliamentary party, let alone by the Commons as a whole. We saw this with PMs Johnson and Truss: it did not make for happy or effective government. Second, it gives a prime minister elected in this way a personal authority that fits poorly with the rest of our system, which is based on accountability to Parliament and on a Cabinet that should function collectively.

Sure, the theoretical notion of the prime minister being the first among equals often is just that: theoretical. But I myself witnessed, as a Cabinet Office official, Tony Blair going around his Cabinet table asking each member for their view on a vitally important matter — and significant dissent would have changed things. It was clear at the time of the Iraq war in 2003 that the war decision needed to get past Cabinet and, even with Blair at the height of his authority, it wasn’t wholly clear in advance.

That kind of accountability goes awry with a leader whose authority derives only from a party leadership election, not endorsed by a General Election. The parliamentary basis of accountability gets diluted. Everyone in Cabinet may think the PM’s proposal is bonkers. But if he or she has just been given a mandate from the party outside Parliament, they can’t easily argue. The PM may be generally disliked across the House. But short of a major rebellion in the ruling party, Parliament is stuck with him or her.

This highlights a further drawback. If they do get rid of the new prime minister, who chooses the replacement? Not the Parliament to whom the government is supposed to be answerable — but the same outside electorate who landed it with the dud (or, at the least, the unacceptable office holder) in the first place. Something has clearly gone wrong.

How could this be improved? One small change, with a small but potentially positive effect, is in place in the Scottish Parliament, the Welsh Senedd — and the Irish Dáil. A new first minister, or Taoiseach, has to win a parliamentary vote to be confirmed in office. That may not normally make much difference, especially if a party has a clear, disciplined majority. But with a minority government or with coalitions in place, it may give a chance to open up bargaining towards someone more acceptable.

It is marginally better than Westminster’s present system, which allows an unpopular new prime minister to take office without parliamentary support. This happened most recently when Mrs May arguably misled the then Queen by advising her that Mr Johnson was likely to command a majority of the House and should be invited to form a government. It didn’t look as though he did command that majority and, indeed, he did not manage to win any major votes before deciding to call an election.

This raises the question of whether the person elected party leader should automatically become prime minister. In Germany, the two main parties separate the party leadership roles from the chancellorship and, out of office, both involve their full membership in choosing both roles. However, if a sitting chancellor resigns, the selection of a successor is determined among members of the parliament.

The Labour Party used to make this distinction. In opposition, the Shadow Cabinet was elected until 2011 by the National Executive (not a parliamentary body); but in government the prime minister had a free hand.

The arguments for letting the parliamentary party of a ruling party choose a new prime minister seem to me overwhelming, both practically and in principle. The chances of it happening are remote. Party members, in any party, are unlikely to give up the power that they have to elect their leaders. Bringing about change when, as now for Labour, a vacancy is a possibility clearly gets too tangled up with immediate politics to be feasible. But it is something that deserves more active debate, to enable, or force, parties to start considering it.

Failing this, MPs need to remember that a prime minister is answerable to them. It would take courage for them to suggest that they know better than their party membership but there have been recent episodes when they clearly did, and failed to act. Just maybe in future, they might get braver. Meanwhile, and more likely, we may have to rely, during coalitions or minority governments, on other parties making demands about whom they are willing to support as PM. This may be crude but might achieve some restoration of parliamentary authority.

But let’s get the debate going.

• George Fergusson was Governor of Bermudafrom 2012 to 2016 and is now a retired senior British diplomat who writes on foreign affairs. This article is reproduced by arrangement with The Herald in Glasgow, where it first appeared