It may not be the crunch, but it is surely the beginning of the crunch. Thursday’s Commons debate on borders and customs regimes after Brexit is a crucial opportunity for MPs to fire a very clear shot across the government’s bows in favour of the softest possible Brexit. The Commons will be debating a motion on Thursday, not a piece of legislation – the latter opportunity will come when the trade bill and the EU withdrawal bill are again discussed by MPs next month. Yet this week’s motion is anything but trivial. It supports frictionless post-Brexit trade borders for manufacturers and it insists on continuing alignments across the Irish border. It is moved by the backbench liaison committee of select committee chairs, so it has backing from senior parliamentarians from Labour, the Conservatives, the Liberal Democrats and the Scottish National party. The outcome must send an unequivocal message that we have reached make your mind up time.

As so often in the Brexit process, the government’s approach is to continue to kick the can down the proverbial road. But the end of the road is in sight. With local elections due next week, the Conservatives have imposed a light whip on their MPs. Some select committees, including the Brexit committee, are scheduled to be on overseas visits. It is important, nevertheless, that as many pro-European MPs as possible attend this debate and vote for the motion. Most businesses in Britain, especially in manufacturing, want to remain in the customs union or to join one that is effectively the same thing after Brexit. Crucially, the government has also made a solemn agreement with the EU to maintain a frictionless border in Ireland. Both are essential. This motion must be carried.

The aim on Thursday and next month must be to maximise the pressure on the government to make the best possible compromise with the EU on customs and tariffs, for the sake of both industry and Britain’s commitments on Northern Ireland. Theresa May prefers indecision and brinkmanship. So, at present, does the EU, under strong pressure from France and Germany to make sure Brexit is not rewarded. But the principles at the heart of the argument – and the parliamentary numbers when the bills return – are about to become inescapable.

One further large truth, too often ignored, is that the leavers’ trumpeted alternative of bilateral post-Brexit trade deals – which would not be possible if the UK embraced any version of the EU customs union – is a sham. These deals simply don’t exist. Liam Fox, the international trade secretary, has drawn an embarrassing blank as he tours the world touting for deregulatory deals. MPs face a choice between a fantasy future and a solid commercially secure future based on existing arrangements. It would be phenomenally irresponsible to opt for the former against the latter. It’s a no-brainer.

Some ministers have floated the idea of making this issue a confidence vote. That shows how vulnerable they are. It is why Thursday’s initial skirmish matters. MPs need to increase the pressure on the government as the Lords amendments to the Brexit bill pile up. Sooner or later Mrs May is going to have to get real about the business case, the Northern Ireland case, and the parliamentary numbers. She also needs to get real about the compromises that will be required with the EU if this is to be made to work. Mrs May has got away for too long with promoting the pyrrhic freedom of a fantasy Brexit. That no longer washes. It is compromise time now. Mrs May can’t duck it. And MPs must ensure that she does not.