With the wonderful benefit of hindsight, the great moments of political choice can take on a deceptive inevitability. Yet in real time, these moments that make or break governments – such as Sir Robert Peel’s repeal of the corn laws, which split the Tories for a generation – are more typically the hard-fought climax of processes that follow a circuitous and up-and-down path, with the key decision put off until it is inescapable.

The moment of decision for Theresa May over Brexit is following this pattern too. The route has meandered for nearly two years through negotiations, summits, law-making and elections; this week’s English elections will be part of the context too. The climax has been long predicted and frequently postponed. To judge by what Downing Street said this week, as ministers prepared for Wednesday’s cabinet committee on customs arrangements with the EU, it may well be pushed back yet again by a few weeks. Yet the moment is nearing all the same.

When it finally arrives it will be bitter and destructive, above all for the Conservative party. But the Brexit terms were always going to have to be faced in some form. That form, it now turns out, will focus on the customs arrangements.

Much of the commentary presumes that the outcome remains in serious doubt. Yet it seems increasingly likely that the UK will seek to remain in some form of customs association with the EU. Whether the EU will agree is, however, another matter.

May has already grappled with this question twice during the Brexit negotiations. Both times it has come up in the unavoidable context of the Irish border. Both times, in December on the divorce terms with the EU and in March when a transition agreement was made, May signed up to a soft border in Ireland. The argument in cabinet this week is likewise about May’s hope of another accommodation with the EU in the form of a “customs partnership”, or its more defiant alternative, the technologically driven “maximum facilitation” or “max-fac” option.

The Guardian view on Brexit and the Irish border: alchemy fails again | Editorial Read more

It is certainly true that the terms of the earlier deals contained significant elements of fudge. It is also true that the UK side still talks about technological max-fac solutions that the EU dismisses as fantasy. Nevertheless, unless May now abandons those two earlier agreements and unless the EU takes a much gentler line than it has done, there are only two realistic outcomes for Britain to choose from while avoiding the return of a hard border in Ireland.

These outcomes are, first, the UK remaining in some form of continuing customs union with the EU or, second, Northern Ireland remaining in the customs union while Britain leaves it. The latter is opposed by Northern Ireland unionists and May dismissed it as recently as Wednesday when she said at prime minister’s questions that an Irish Sea customs border was something no UK prime minister would agree to.

This leaves UK membership of the customs union – or something on similar lines – as still much the most likely outcome of the internal British political argument. Why? Because the EU has shown no interest in, and is under little internal pressure to explore, either of the alternative options being debated by British ministers, and the arithmetic in parliament is tending that way too.

Q&A What is a customs union? Show Hide A customs union means that countries agree to apply no or very low tariffs to goods sold between them, and to collectively apply the same tariffs to imported goods from the rest of the world. International trade deals are then negotiated by the bloc as a whole. For the EU, this means deals are negotiated by by Brussels, although individual member state governments agree the mandate and approve the final deal. The EU has trade deals covering 69 countries, including Canada and South Korea, which the UK has been attempting to roll over into post-Brexit bilateral agreements. Proponents of an independent UK trade policy outside the EU customs union say Britain must forge its own deals if it is to take advantage of the world’s fastest-growing economies. However they have never explained why Germany manages to export more than three times the value in goods to China than Britain does, while also being in the EU customs union. Jennifer Rankin

In the House of Lords, Tory peers have voted with the opposition parties to demand a continuing customs union. Meanwhile, in the Commons pro-European Tory MPs have put their names to pro-customs union amendments to two government bills. These would carry forward last week’s backbench debate on the same subject, on which a resolution of support for continued customs union membership was adopted without opposition. None of this guarantees that parliament will take a similar line when the votes come up in a few weeks’ time. But none of the other options is any likelier. At present, the numbers are close but in favour of the amendments. All the opposition parties (bar a small number of Labour leavers) plus the 15 or 20 declared Tory rebels would normally be enough to either defeat the government outright or to force it to accept defeat in advance, as it did this week on tax haven controls.

The very fact that last month the government seemed to be flirting with making these into votes of confidence makes clear that they think they will lose them. Yet behind that admission there is a deeper and more stubborn truth. There is not a majority in the Commons for a hard Brexit. And there is no alternative prime minister to May who can deliver one, either. Short of a general election or a collapse of the negotiations, the only outcome of this process is a soft Brexit of some kind. If we see May’s purpose as being to obtain the least divisive soft Brexit for the Tory party, we are much closer to understanding what is happening.

Yet there will be divisions. Some Tories will reject any soft Brexit, although no cabinet Brexiteer has said it is a resignation matter. But the issue need not bring May’s government down. Jacob Rees-Mogg’s Eurosceptic sabre-rattling over the customs issue this week is a bluff she will have to call.

In the end and with no small irony, the pivotal issue in Brexit is the same one that in different ways has so haunted British politics for more than two centuries – justice for Ireland. Today, peace and prosperity in Ireland depend upon a soft Brexit for which there is a parliamentary majority. A hard Brexit, with a hard Irish border, is clueless, delinquent and playing with fire, as Chris Patten put it in the Lords on Wednesday. It was a similar choice for Robert Peel. In February 1846, as the corn law repeal process neared its climax in the wake of the potato famine, Peel told his MPs: “It is absolutely necessary before you come to a final decision on this question that you should understand this Irish case. You must do so.” Then, as now, it was the right advice.

• Martin Kettle is a Guardian columnist