Will Sunday's Brexit history end up in the dustbin?

Because Theresa May's Brexit deal has been so long in the coming - almost two and a half years - and has been so comprehensively trailed and leaked, Sunday's formal ratification of the terms of our departure from the EU and the shape of our possible future relationship with it feels like the mother of all anti-climaxes.

But cynicism and lethargy are to be resisted: that ratification really matters.

Because - at last we have THE DEAL.

Until today, everything about Brexit was presumption, speculation, rumour and hypothesis.

Finally, we know what Brexit means to a prime minister who had no other job but to find out what it means.

Her deal has a decent number of known knowns: the £39 billion divorce bill, that EU migrants living here and British migrants living in the EU can stay where they are without detriment, that free movement to the UK from the rest of the EU would end, inter alia.

It also has a significant number of transparent known unknowns: that the border on the mainland of Ireland could be kept open by the imposition of different rules in Northern Ireland from those applying in Great Britain, that the UK could remain a non-voting member of the EU for a few years yet, that the permanent new terms of access for UK businesses to the EU's single market are yet to be settled and will be connected to quite how much the UK is prepared to be bossed by the EU.

Donald Tusk, Jean-Claude Juncker and Michel Barnier at the Brussels summit. Credit: AP

The big point is that although the prime minister insists - and will continue to insist - that her deal makes real the promises of Brexiters that Britain will take back control of its borders, money and laws, in practice only when it comes to borders and immigration does her agreement concretise those aspirations in a way not open to dispute and even ridicule.

Which means that MPs will have to compare apples with pears - sovereignty arguments with economic ones with social and cultural ones - when deciding in the next two and half weeks whether Mrs May's Brexit should live or die.

So for all my insistence that what the EU and May agreed today is history, there is still a serious prospect that today's history will end up in the dustbin.

That will be decided on 12 December, which is when I am reliably told MPs will have their so-called "meaningful" vote on all this.

By then (and probably later this week) they will have been shown analysis by the Treasury that - just in terms of the prosperity or economic growth of the UK - Mrs May's Brexit is significantly better than a no-deal Brexit and significantly worse than remaining in the EU.

These prognostications - for those who believe them honest rather than the rantings of an anti-Brexit establishment - will provide some ability to measure apples against apples: the lower growth of Mrs May's Brexit would probably reduce tax revenues flowing to the Treasury by more than the quantum saved by the Treasury in reducing what it contributes to the EU's coffers.

But even for those who trust the Treasury's forecasts, that cannot be the end of it: none of those fiscal or tax numbers matter if your pre-occupation is with the possible wedge driven between Northern Ireland and Great Britain by Mrs May's agreement.

Will the prime minister get her Brexit deal through Parliament? Credit: AP

So will Mrs May have her day?

My presumption is that her deal is highly unlikely to carry - for the reason that it has been overseen by a Remainer, the PM, and confected largely by Remainers in Whitehall. A Brexit made by Remainers - a Brexit made by those whose hearts could never properly be in it - will struggle to satisfy sufficient numbers of either Brexiters or Remainers.

If she has hope it will be to somehow persuade a majority that the alternative to her putative mediocrity of a deal is the supposed catastrophe of no deal.

The problem however is that significant numbers of her Cabinet, and certainly a majority in Parliament, are utterly convinced no deal would be a catastrophe of wartime proportions - leading to food and medicine shortages and factory shut downs.

So at least five members of the Cabinet - the Chancellor Philip Hammond, the de facto deputy PM David Lidington, the business secretary Greg Clarke, the work and pensions secretary Amber Rudd and the Lord Chancellor David Gauke - would quit if Mrs May's response to her own deal being incinerated was to adopt no-deal Brexit as official policy (by the way, contrary to newspaper reports, these are not a formal "gang of five"; they have never met together to discuss tactics; but they are more of a single unified organism than their pizza-scoffing Brexiter ultra opponents).

And MPs would take whatever parliamentary action was permissible to frustrate a no-deal Brexit.

Which means according to senior ministers, the PM would be left with only one option: to seek to delay the moment of Brexit, to announce (after all that) we are not leaving on 29 March 2019.

Labour and the DUP have indicated they will not support the deal. Credit: PA

I am reliably informed by senior officials in Brussels and in foreign capitals that a request to postpone Brexit is both half anticipated and would be sympathetically received.

But there would be strings.

The delay could be no more than a couple of months, because there is an absolute horror of the UK participating in elections for the European Parliament at the end of May.

And EU governments would want an assurance that the extra time would allow the UK to achieve a settled position, either on the Brexit it actually wants, or on no Brexiting at all.

Here is the nightmare, for the UK and the EU, for them and for us.

It is patently, obviously, maddeningly clear that there is no settled position in Parliament on an optimal Brexit.

That leads a growing number of MPs to muse that the best thing may be to cancel Brexit altogether.

Some tell me that as a nation we should risk the humiliation of apologising to the EU - and the world - and just rescind our "Article 50" request to leave, even if that entailed seeing the EU in court over it.

It would not mean cancelling Brexit forever. But it would mean buying back the months or even years needed to decide a proper and decent future relationship with the EU.

But more MPs would blush to do so without seeking the permission of the electorate in another referendum - but it is by no means clear that the British people are less divided and more certain about all this than their elected representatives.

When Parliament and people are divided, there is no higher authority to consult.

This is a proper a mess - and, no excuses, one of our own making.