Late on Tuesday, the Labour MP David Lammy tweeted these words: “I just want to run through the corridors screaming ‘wake the **** up people’”. He’s right to feel that way. I know that British politics is meant to be conducted – by politicians and commentators alike – with polite restraint, all hints and understatement, but when madness surrounds us, then it makes sense to get mad.

And make no mistake, what we are witnessing is a collective insanity. We now learn that the British army will put 3,500 troops on standby to manage the fallout from a no-deal crash-out from the European Union on 29 March. The health secretary has announced that he has become the world’s biggest purchaser of fridges, so that the UK will have sufficient storage capacity for perishable medicine. In the same vein, he has chartered a dedicated NHS plane to ensure medical isotopes – vital for cancer treatment – can be flown directly from Holland to Britain in the event of a no-deal exit.

All this is the work of ‘responsible government', ministers say. It would be, if they were bracing for a hurricane

The flights will come from Maastricht – yes, the same Maastricht that gave its name to the treaty which drove Europhobic Tories to fury a generation ago – at an estimated cost of £30,000 per flight. Meanwhile, the cabinet has drawn up plans to ration space on cross-channel ferries: it assumes that once France introduces checks and customs controls, as it will if we crash out, trade on the essential Dover-Calais route will collapse by 87% – triggering instant food shortages. No wonder all five organisations representing British business have joined together to say their members are “watching in horror”.

All this is the work of a “responsible government”, ministers say. It would be, if they were bracing for a hurricane to hit these islands. Or a mass outbreak of ebola. But this is not same natural disaster, inevitably heading our way. This is a self-inflicted catastrophe, one we could easily avoid.

Instead of spending £4.2bn on no-deal contingency planning – precious taxpayers’ money that could be spent on nurses or teachers, or on ensuring that no one sleeps rough at night and no child goes to school hungry – Theresa May could simply announce that under no circumstances will the UK leave the EU with no deal on 29 March. She could explain that if her own plan is not approved by parliament, then she will extend or rescind article 50 to give Britain more time.

Indeed, it should be obvious. No deal would represent an unconscionable act of self-harm, a country punching itself hard in the face, visiting upon itself the kinds of trials and hardships associated with war – and no government should even countenance such a criminal act. That should be the reddest of May’s famous red lines – a commitment not to inflict catastrophe on her own country.

And yet, on it goes, each day getting worse. Today we have the government announcing its post-Brexit immigration policy, though it has merely announced more “consultation” on the most neuralgic questions. But what’s clear is that this government would like to close the door to EU migrants who are not earning an above-median annual salary. Which means cutting ourselves off from the care workers, nurses, junior doctors and others we desperately need. Once again, Britain shows all the sign of a nation bent on inflicting pain on itself.

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Our politicians, of every stripe, should be devoting themselves entirely and without restraint to preventing this calamity. There should be no other consideration – not of personal ambition, jockeying for leadership, nor of partisan advantage, working out whether this or that move will boost their party’s electoral prospects. None of that matters when a national emergency is looming. The only purpose of politics at this moment is averting it.

Which is why Conservatives such as Nick Boles are right to say they would vote to bring down their own government to avoid no deal, if that’s what it takes. And why Lammy is right to want to run screaming through the corridors, sounding the alarm. It’s unconscionable that Tories are tolerating May’s games, as she delays the meaningful vote that could trigger the parliamentary sequence that might halt this madness. And it is no less unconscionable that Jeremy Corbyn keeps playing his own games, avoiding the formal vote of no confidence in this horror show of a government which, once done, would compel Labour to pursue other ways out of this quagmire. He is meant to be the leader of the opposition: his job is to oppose. And nothing needs to be opposed more urgently than this coming catastrophe.

• Jonathan Freedland is a Guardian columnist