“Crunch time is coming for the prime minister,” Keir Starmer, Labour’s Brexit spokesman, tells the BBC’s Andrew Marr. But, then again, when isn’t it? Theresa May is more familiar with crunch time than an overworked gravel salesman.

What gives force to Starmer’s claim is that the ordeal now facing the prime minister is of a different order and character to the daily miseries that have afflicted her since the general election last June. Ostensibly a principled challenge by Jeremy Corbyn to her position on Brexit, it is really a ruthless political challenge to her position, full stop.

You can be sure that the Labour leader’s speech on Monday will disappoint those in his party who hoped he would embrace full, continued, unambiguous membership of the EU customs union. Corbyn’s longstanding ideological recoil from the EU will see to that, as will the unrelenting pressure from Eurosceptic Labour MPs and some of his closest advisers, who believe that a sellout on Brexit will be fiercely punished by working-class voters.

He can wait for this government to collapse under the weight of its own contradictions. Or he can strike

So don’t expect a tearful declaration that he does, after all, love Big Brussels. There has been no epiphany, no sudden volte-face. Yes, his mind has been focused by the risk of a hard border between Northern Ireland and the Republic of Ireland and the consequent mortal threat to the Good Friday agreement. But the shift he is about to announce is primarily political rather than doctrinal in inspiration.

The potential to cause havoc in the Commons was demonstrated by the success in December of Dominic Grieve’s amendment to the European Union withdrawal bill giving MPs a legally guaranteed vote on the Brexit deal. But there are only so many procedural issues upon which Labour can join forces with remainer Tories if the bombardment is to be sustained.

This is what Corbyn is doing. By shifting the needle even slightly on the question of a future customs arrangement with the EU, he will – at a stroke – create the germ of a cross-party rebel alliance bound not only by an insistence on the supremacy of the Commons but also by shared policy values. This is a hugely significant moment.

Quick Guide Main points of agreement in the Brexit deal Show EU citizens EU citizens in the UK and UK citizens in the rest of the EU have the right to stay. Rights of their children and those of partners in existing “durable relationships” are also guaranteed.



UK courts will preside over enforcing rights over EU citizens in Britain but can refer unclear cases to the European court of justice for eight years after withdrawal. Irish border The agreement promises to ensure there will be no hard border and to uphold the Belfast agreement.

It makes clear the whole of the UK, including Northern Ireland, will be leaving the customs union.



It leaves unclear how an open border will be achieved but says in the absence of a later agreement, the UK will ensure “full alignment” with the rules of the customs union and single market that uphold the Good Friday agreement.



However, the concession secured by the DUP is that no new regulatory barriers will be allowed between Northern Ireland and the rest of the UK without the permission of Stormont in the interest of upholding the Good Friday agreement. Money There is no figure on how much the UK is expected to pay but the document sets out how the bill will be calculated – expected to be between £35bn and £39bn.



The UK agrees to continue to pay into the EU budget as normal in 2019 and 2020.

It also agrees to pay its liabilities such as pension contributions. Other issues The two sides agreed there would be need for cooperation on nuclear regulation and police and security issues.

There was an agreement to ensure continued availability of products on the market before withdrawal and to minimise disruption for businesses and consumers.

I sympathise with the 80 senior Labour figures who signed a statement issued to the Observer urging him to go further and to back membership of the single market. They are right that Britain’s economic interests would be profoundly damaged by our departure from that hard-won commercial arrangement. But they misinterpret what Corbyn is doing, and why. His conversion is not to soft Brexit, but to a hard knock on the door of No 10.

With ever greater clarity, it is dawning upon Labour strategists that there may indeed be a “Corbyn moment” – which is to say, a limited period of maximum electoral opportunity. For an opposition, there is always a case for playing it long, campaigning in more locations, fleshing out the difference you will make to sceptical voters. But time can be a foe as well as a friend – never more so than in the capricious age of novelty politics and fickle social media.

Last summer belonged to Corbyn, in mood, tone and electoral trajectory. But volatility, by definition, destroys that which it creates. Etched into the waves of euphoria is the certainty of the opposite emotion. The only variable is the time span separating infatuation from rejection. It could be years – but not necessarily so. Remember Cleggmania? Remember May’s apparently unbreakable command of the political stage in late 2016? In modern politics, the only mantra worth repeating is: this too shall pass.

So what are Corbyn’s options? He can hold tight and wait for this jerry-rigged government to collapse under the weight of its own contradictions. Or he can strike.

The first option is proving risky, as May’s lonely talent appears to be a capacity to turn her party’s confusion and indecision to her advantage. The cabinet’s commitment last week to pursue a Brexit deal based upon “ambitious managed divergence” is yet another tribute to her dreary genius. The wording is completely meaningless in all respects – except that it has extended the Tory truce a little longer.

And this is what May does; all she does, in fact. She leads by procrastination. She insists upon inaction. She is the woman ordering endless Ubers to take her to the place where she already stands.

The alternative option for the Labour leader is to force the pace, and to do so in parliament. In 1992-93, John Smith paved the way for Tony Blair’s victory by tactical alignment over the Maastricht treaty with the Tory Eurosceptic “bastards” – a Commons rebellion that did terrible damage to John Major’s authority.

A quarter of a century later, Corbyn has a comparable opportunity to make common cause with the Tory remainers: not always, but often enough to tip the Conservative tribe from paralysis to panic. His new position on the customs union is meant to light the fuse that leads – however circuitously – to a confidence vote in the PM and another general election. It is not a strategy without peril. It could fail, and embarrassingly so. But it is still, as Michael Corleone would say, the smart move.

• Matthew d’Ancona is a Guardian columnist