Imagine you are sitting in a much loved and overused chair. Your habit is to lean back on it a bit while holding on to the table in front of you, putting pressure on the rear two legs. You’ve got used to this slightly awkward position. It has started to feel quite comfortable.

One day, while enjoying your customary lean, you notice a creaking sound. It is quiet, but distinct. The pressure on the chair’s legs is becoming audible. You lean forward quickly and forget about it. But the next day, you lean back again. This time, the creaking is loud. And before you have time to push yourself forward, one of the back legs snaps in two and you find yourself on the floor, your beloved chair lying ruined underneath you.

You are the Labour leadership. The chair is your Brexit policy. And this week, you heard the first creaks.

We have all got used to the ‘constructive ambiguity’ of Labour’s position. The party is essentially pro-Brexit. It voted to trigger Article 50, and maintains to this day that if it were in power it would negotiate its own withdrawal deal with the European Union. It has done this primarily because a significant chunk of its electorate – although only about a third – voted to Leave. And yet throughout this tortuous process, it has sought to placate the two-thirds of its electorate plus the vast majority of its membership who voted Remain by signalling its general unhappiness with Brexit. Labour was against the Prime Minister’s deal, and for the maximum parliamentary scrutiny of Brexit. And its conference position, set out in October last year after an interminable meeting involving 300 delegates, explicitly left a people’s vote on the table.

All this ambiguity has undoubtedly borne fruit. It got the party through the 2017 General Election handsomely, where Labour defied expectations at least partly because it seemed like the only viable vehicle for anti-Brexit sentiment. And it has allowed Labour to sit back while the Conservative Party tears itself apart. Despite the awkwardness of their position, they have long been comfortable with it.

But earlier this week, as the Government presented its Immigration Bill for Second Reading, the creaking was audible. This is the Bill which will end the free movement of EU nationals into the United Kingdom, thereby starting a new, more closed and more hostile chapter in our nation’s history. And until late in the day, Labour’s assumed and then confirmed position was that it would abstain on the Bill. Cue much consternation not only among known enemies of the leadership, but among some of its most prominent backers. Guardian columnist Owen Jones and Novara Media journalist Ash Sarkar and many others weighed in to express their dismay that Labour would be complicit in this moment of hostility to foreigners.

And then came the volte face. At the last minute, Labour MPs were whipped to oppose the Immigration Bill. The Labour leadership would not make the same mistake which Harriet Harman made in 2015 when she whipped MPs to abstain at second reading of the Government’s hated welfare reforms. That move fundamentally shifted the politics of the unfolding Labour leadership saga, as Corbyn (who did not abstain) was able to paint the other contenders (who did) as triangulating schemers interested only in occupying the putative centre ground. No, they could not allow themselves to be brought down from the left. They have not yet accepted that the puritans have become the triangulators – that their Brexit policy, and the parliamentary positions which logically entail from it, make them every bit as compromised as any other serious mass-membership, catch-all political party.

But it doesn’t end here. At some point this issue will come back. The party is apparently committed to ending free movement – so what is the basis for opposition to a Bill that does almost nothing but end free movement?

All this time, while Labour has been leaning back on its chair, the moments of pressure which might have broken up their ambiguous Brexit position have not really made themselves known. Triggering Article 50, navigating the labyrinthine process of Brexit – it’s all abstract, at least one step removed from its real effects. But the pressure has been building up. And immigration reform? On this issue, the reality is inescapable. Ending free movement will have immediate and stark effect on millions of people, in Britain and beyond. The real effects of Brexit can no longer be ignored. The creaking is getting louder.

This week, the constructive ambiguity was back in place. Labour supported the Cooper amendment to provide time to extend Article 50 and avoid no-deal, but made it clear it wouldn’t sanction a full nine-month delay. Again, at this level of abstraction, the uneasy coalition underpinning Labour’s position did not break apart at the seams.

But how long until it does? When will the chair legs break? Will it be at Third Reading of the Immigration Bill? Or will the real impact of Labour’s complicity in Brexit be exposed at some moment of crisis in the coming weeks – perhaps as we head over the no-deal cliff edge?

As with so much of Brexit, nobody knows. But make no mistake: constructive ambiguity cannot be maintained forever. The pressure is building. And unless they make a decisive move in one direction or the other, sooner or later the Labour leadership will find themselves on the floor, their comfortable chair lying ruined beneath them.

Will Brett is the director of research and campaigns at Global Future