Under the conventions of our constitution, there must always be a Prime Minister.  So were Boris Johnson to be forced out, who would replace him?

The answer should be: a new Conservative Party leader.  But while the appointment of a new Prime Minister can be agreed in few days – even hours – a Tory leadership election as we know it would take several weeks.

The most economical solution would be for Conservative MPs to agree a single leadership candidate who would become Prime Minister at once, and whose sole name would then go forward to party members for endorsement.

But who would this person be?  I find it hard to imagine anyone – Rishi Sunak, Liz Truss, anyone else – commanding universal consent among Tory MPs. Or each other.

An impasse could be avoided were it agreed that Johnson would stay on as Prime Minister until a truncated leadership contest with a swift Parliamentary and membership stage was concluded.

However, this might not be practicable, were public feeling to require his immediate resignation (as might well be the case).

Another means of avoiding a crisis would be for the Deputy Prime Minister to make it clear that he would not be seeking the Conservative leadership.

He could then serve as an interim Prime Minister for a few weeks until the new Conservative leader kissed hands – (assuming that he or she could command a majority in the Commons as would surely be the case).

Were he unwilling to do so, the Cabinet could then seek another of its members who would.  But it might not be able to agree on a single name.

The Parliamentary Party would also need to be consulted.  The whips would presumably take “soundings”.  So would the 1922 Committee Executive.  Senior members of the voluntary party would want a say.

That sounds like a lot of Tory cooks with ladles in a murky broth.  Furthermore, there is no rule book for soundings – no clear procedure that commands consent.

Keir Starmer would presumably call a vote of confidence in the Government.  The Commons would be fissile.  There would be talk of a constitutional crisis.

I peer into this fog not to suggest that the Conservative Party shouldn’t walk into it, but to point out that it should be aware of the potential consequences if it does.

Johnson’s best hope is to apologise today, waits for the Sue Gray investigation to conclude, and hope that it doesn’t offer him with the political equivalent of a revolver and a bottle of whisky.

He would then reshuffle his political and civil service team in Number Ten after the inevitable resignations in the wake of the investigation, and lie low until public anger abates.

Then, Michael Gove unfurls his levelling up White Paper. Rishi Sunak finds a compensation package for the poorer losers of the cost of living squeeze.  The Prime Minister lurches on towards the spring’s local elections.

But does all this sound at all credible?  What if Sue Gray does indeed hand Johnson that gun and bottle?  What if the Met then feels (however reluctantly) it must make further enquiries?

What if the Cabinet revolts in the wake of the Gray report?  Or does so anyway?  If Ministers start to resign?  If backbenchers won’t vote against Opposition motions of censure?

If Graham Brady gets 54 letters demanding a ballot of confidence in Johnson?  Above all, how likely is it that public anger will abate?

If the Prime Minister doesn’t quit now, might the damage to the Conservative Party be greater that if he goes later?  And what of more consequential harm – to the coherence and operability of government?

It may be that Johnson is able to confirm that he didn’t break the lockdown rules that applied to him no less than everyone else.  Or that there is some explanation that has not yet been forthcoming.

Neither seem to be the case as I write.  Would it therefore be better for the Party to take the hit of a Prime Ministerial resignation now – and plunge into the uncertainties that would follow – rather than prolong the pain?

Or is the least bad course to cross fingers, and hope that “the greased albino piglet” escapes again?  After all, he’s defied the odds many times before.  And won some of the very Tory MPs who would oust him their seats.

But that was in partnership with Dominic Cummings.  The Prime Minister broke it off.  His Government has not been quite the same since. Now Cummings is serving his dish of revenge white-hot.

My sense before the New Year was that a leadership challenge had become more likely than not.  Today’s Prime Minister’s Questions may tell us more.  Will a Conservative MP call on him to go?  (Or more than one?)

The piglet has wriggled free many times before.  But he is cornered in a cul-de-sac and the butchers are whetting their knives.