Gloss it as they may, if EU leaders force a Brexit delay of a year on the UK, contrary to the request from Theresa May - as the EU president Donald Tusk wants - then they will have made a momentous judgement that will cause an earthquake, for us and them.

They would be sending a signal that they have lost all confidence in the UK prime minister - and probably any UK prime minister - securing parliamentary approval of the Brexit divorce settlement, the Withdrawal Agreement, that they painfully negotiated over two years.

The point is that a delay of a year would remove all pressure on equivocal MPs of all, or any, party to agree a Brexit compromise. And those equivocal MPs are the majority. Any urgency to the current talks with Labour - which already looked like a fig leaf for both sides - would dissipate.

Although the theoretical risk of a no-deal Brexit would remain, to all practical purposes it would have vanished.

Labour 'could do well' in the European Parliamentary Elections. Credit: PA

And that would have an enormous impact on the nature of debate within a parliament whose members are largely not ideological Brexiters.

There would no longer be any kind of meaningful deadline to finding a sub-optimal fudge to avoid what most MPs see (rightly or wrongly) as the economic and security disaster of a no-deal Brexit.

It would be embarrassing for the UK to participate in the European Parliamentary Elections, but only for the Tories would it be worse than an embarrassment - Labour would probably do well under a PR voting system that would allow those who want a people's vote to back an opposition party that has moved in their direction.

So a year's Brexit delay would in effect give permission - emotional and psychological - for a more open, honest and less time-sensitive debate about what kind of Brexit the British people actually want, and whether they in practice want Brexit at all.

At that juncture, a further referendum would move from being the least likely outcome to the most likely.

Michel Barnier (left), the EU's Chief Brexit Negotiator, meets with Taoiseach Leo Varadkar in Ireland on Monday. Credit: PA

Which doesn't mean Brexit would never happen. But it certainly very much reduces the probability of it ever happening.

This is an outcome that would suit Tusk, the Republic of Ireland and perhaps most EU countries. It is the precise opposite of the prime minister's ambition.

Paradoxically, it would also suit Northern Ireland's DUP, whose 10 MPs sustain the Tories in office, because it would remove - or at least massively reduce - the perceived threat to the union between Northern Ireland and Great Britain.

Such a delay would not necessarily lead to the collapse of the government and a general election. But it would inevitably lead to the imminent departure of Theresa May - because the only (if remote) prospect of securing some kind of Brexit for the majority of Tory MPs who say that's what they want would be to find a way to replace her forthwith.

Donald Tusk wants to give the UK a Brexit extension of a year. Credit: AP

So for the avoidance of doubt, if EU leaders force a year's Brexit delay on the UK, even if garnished with the option for the UK of being able to leave earlier (if miraculously preparations for Brexit can be achieved earlier), the EU would be voting to usher in a new British prime minister and probably to extinguish Brexit.

Yesterday I assumed - guided both by their officials and my experience and judgement - that EU leaders would not wish to make a decision that had such historic consequences.

I may well have been wrong - partly because May herself has already signalled her early departure, and because she has formally allowed a Brexit role for an opposition Labour Party whose commitment to leaving the EU has been weakening by the hour.

Or to put it another way, EU leaders may well be able to reassure themselves that a long Brexit delay is going with the grain of history, not changing it.