The term “Project Fear” must be expunged from the respectable political lexicon. Now that we know the full detail of the government’s Operation Yellowhammer assumptions about a no-deal Brexit, these two words – scornfully applied for three years to all warnings about Brexit – have no meaningful place in the present political emergency.

“There’s a lot of scaremongering around,” declared the energy minister, Kwasi Kwarteng, on Sky’s Sophy Ridge on Sunday show, in dismissive response to the Sunday Times’ reports on the leaked Yellowhammer document. Really? These, after all, are the government’s own assessments, not the panicked assertions of ultra-remainers.

In the event of a no-deal exit, there are likely to be serious shortages of fuel, food and medicine; disruption at the nation’s ports; civil disorder; increased poverty; and a hard border with Ireland. To be profoundly and vocally concerned about such a prospect is not “scaremongering”, but a basic civic responsibility.

If you look to the heavens and see a UK-sized breeze block plummeting towards you, it is not “alarmism”, or – as Boris Johnson would put it – a failure of character and confidence, to try to avert its descent. Only last month, he was insisting that “a lot of negativity about a WTO [no-deal] Brexit has been wildly overdone …The planes will fly, there will be clean drinking water and there will be whey for the Mars bars because where there’s a will there’s a way.” What fun! Except the joke is wearing very thin.

Indeed, as the “Project Fear” slur diminishes in impact, it is intriguing to observe the prime minister and his acolytes preparing its successor: the “Great Betrayal” narrative. In a letter to Philip Hammond and 20 other Tory MPs seeking to thwart no deal, leaked to the Mail on Sunday, Johnson alleges that “it is as plain as a pikestaff that Brussels – or the EU27 – will simply not compromise as long as they believe there is the faintest possibility that parliament can block Brexit on 31 October”. In other words: if no deal happens, it will be because the EU saw no need to renegotiate. It will be your fault, Spreadsheet Phil.

This is spectacularly disingenuous. There are absolutely no signs, coded or otherwise, from the EU27 that, dazzled by Johnson’s brio and joke book, they are ready to strike a new deal in the time remaining before 31 October.

It is he who insists that this deadline cannot be shifted, “no ifs, no buts”. It is he and his fellow Brexiteers who led the charge against Theresa May’s withdrawal agreement, making it impossible for the Commons to agree on an alternative. We are in this predicament because of the serial dishonesty, manoeuvring and lust for power of those who insisted that leaving the EU would be easy. This may be what Dominic Cummings, the PM’s senior adviser, would call an “unhelpful narrative”. But it’s also the truth.

Play Video 1:41 Jo Swinson says Corbyn cannot become temporary PM – video

What matters now is how those who want to prevent a historic disaster respond to the challenge; how imaginative they are prepared to be; and whether they are ready to put nation before party, and the interests of subsequent generations before narrow political advantage. Johnson and Cummings are determined to get the UK out on 31 October “by any means necessary”. Are their disaggregated opponents equally committed?

The question answers itself. I deplore those who are ready to vandalise the future of this great country just to satisfy an ideological itch. But it is also hard not to be exasperated by the fastidiousness of their opponents – a squeamishness that is fine in times of relative political calm, but an unaffordable luxury at moments such as these, when the stakes are vertiginously high.

Let us address the bearded elephant in the room. I think I have made it fairly clear that I do not want Jeremy Corbyn to become prime minister and that I firmly believe he is not up to the job. But – if Johnson loses a vote of no confidence in September, triggering a 14-day period in which, by law, any MP can seek to form an alternative government – it is self-evident that Corbyn will be at the heart of any such discussions, ex officio, as leader of the opposition. I might wish that Stella Creasy or Yvette Cooper or Hilary Benn held that post. But none of them does.

As it happens, I doubt that Corbyn could cobble together a Commons majority. But it was foolish of the Liberal Democrat leader, Jo Swinson, and Tory opponents of no deal such as Dominic Grieve and Oliver Letwin, to make their opening move a veto on the Labour leader heading even an interim government committed to an extension of article 50, an early general election and a fresh referendum. All this grandstanding has been a great morale-booster for the no-deal Brexiteers. Who can blame them? The very first signal sent by their opponents was one of disunity, introspection and petty partisanship.

Again, to be clear: I think the chances of a so-called government of national unity being formed have always been slim. The better prospects are for parliament to pass fast-tracked legislation to mandate the government to seek an article 50 extension; and a variety of strategies in the courts, especially a judicial review to prevent Johnson postponing a general election until after 31 October.

That, though, is not the point. As the comedian Dave Chappelle has put it, progress always, inevitably, depends upon a readiness to work with imperfect allies. Those who claim they are committed – really committed – to preventing the outrage of a no-deal exit have yet to prove that they are prepared to do whatever it takes. And time, to put it mildly, is running out.

• Matthew d’Ancona is a Guardian columnist