Sounding the klaxon and warning of imminent disaster is, it turns out, an even older British tradition than you might think. A tour of the spellbinding Anglo-Saxon Kingdoms exhibition at the British Library confirms that back in the year 540, a scribbling sermoniser by the name of Gildas – surely the Guardian columnist of his day – was writing “on the ruin of Britain”. Even before the country properly existed, there were Cassandras to prophesy its demise.

Still, that same exhibition leaves the visitor in no doubt that what will befall these islands in less than 50 days is of epic significance, breaking a thread that has run through our long history. Even in the age of Mercia, the kingdom strained hard to connect with its neighbours in “Francia”, Rome and Ireland. The 10th-century court of Æthelstan was a cosmopolitan magnet to scholars from all over the continent. And need we mention that the Anglo-Saxons were themselves migrants from northern Europe? The Faragiste tendency, which imagines a British past pure and unsullied by the taint of Europe, imagines a past that did not exist.

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So there should be no apologies for clanging the church bells and crying havoc at the prospect of Brexit on 29 March. Viewed one way, that date became a tad less scary this week, thanks to Jeremy Corbyn, whose letter to Theresa May, offering Labour approval for an exit path out of the EU via a permanent customs union, makes a no-deal crash-out less likely. For inching us away from that catastrophe, Corbyn deserves credit. On the other hand, for inching us closer to Brexit happening at all, and with Labour’s blessing, he deserves blame.

This was May’s game all along: to keep dangling the lunacy of no deal over our heads until an agreed Brexit looked sane by comparison.

But it’s not sane. Even the softest, mildest Brexit-with-a-deal represents an act of national folly that would have had Cnut shaking his head in disbelief. Take a look at Labour’s demands – alignment with the single market and on workers’ rights along with a say over EU trade deals – and realise that these are the things we already have under our current deal, namely EU membership. The very best that can be said for Labour’s Brexit is that it will only be much worse than the status quo rather than much, much worse. (Don’t hold your breath to see that line in the next manifesto.)

Facebook Twitter Pinterest Theresa May in Brussels after a meeting with Donald Tusk on Thursday. ‘What if the process of elimination falters and May’s deal squeaks through after all, helped over the line by Labour votes?’ Photograph: Stéphanie Lecocq/EPA

No one can make a positive case for it, listing the concrete benefits it will bring – because it won’t bring any, and certainly not “the exact same benefits” of EU membership that Labour once, delusionally, promised. The government’s own figures show that Brexit in whatever form will make us poorer. (Norway plus would be a less painful Brexit but it would also be a pointless one.) It means pro-Europeans are left banking on May and her party rejecting the Labour offer – fleeing from the customs union proposal because it won’t allow us to make our own trade deals. (As if, solo, post-Brexit Britain will match the clout that saw the EU strike the deal with Japan that came into force this month, creating a massive free trade area that will combine a market of 500 million consumers with the world’s third-biggest economy.)

If May does reject the Labour offer of a joint, bipartisan Brexit, pro-Europeans are banking on Corbyn and John McDonnell finally keeping their word and honouring Labour party policy by backing a second referendum. From the start, the people’s vote strategy has been to wait patiently until all other options are exhausted, at which point MPs, out of alternatives and at the very last minute, will realise the train is about to crash and reach for a referendum as the nearest available emergency cord. That strategy has always been high-risk. What if the process of elimination falters and May’s deal squeaks through after all, helped over the line by Labour votes? For the strategy to work, MPs must not only recoil from no deal, but then decide to flee to the safe harbour not of May’s Brexit deal but a public vote. A lot has to go right for all those things to happen.

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For that reason, I think pro-Europeans now have to make a different move. They can’t wait for the starting gun of a referendum that may or may not ever sound. Nor is it good enough to devote their energies simply to campaigning for such a vote, or to Commons manoeuvres to get one. They need instead to make the anti-Brexit case to the public right now, as if the referendum campaign had already begun. Don’t wait. Put up the posters, book the halls, spend the money right away. Do it now.

The reasoning is simple. So far, pro-Europeans have emphasised the means – a second referendum – over the ends: staying in the EU. That’s the wrong way around. The first task is to persuade the public that Brexit has to be stopped: that leaving the EU will bring ruin, while staying in and making it better is the confident, open, patriotic choice. Only when voters feel that stopping Brexit on 29 March is an urgent priority will they then demand that their MPs find a way to do it, turning to a second referendum as the obvious democratic means. There is a way, but first there must be the will.

Facebook Twitter Pinterest ‘Our ancestors would not recognise today’s Brexiters, so cowed and lacking in courage that they fear they can’t cut it at the top table, sitting as equals alongside France and Germany, but must hide instead in the corner, on their own.’ Photograph: Liam McBurney/PA

The signs are more encouraging than you might expect. The polling sage John Curtice reported today that his poll of polls now has remain leading leave by 54% to 46%, with pro-Europeans way out in front among those who did not vote in 2016 (often because they were too young to do so). More striking is the small but distinct erosion of support for Brexit among those who voted leave last time. Both those groups need the push of a spirited, all-out campaign, rather than 49 more days spent listening to arcane procedural wrangling in the Commons while pro-Europeans politely wait either for Corbyn – a lifelong Eurosceptic who not long ago was calling the EU a “military Frankenstein” that actively desired unemployment – to come onside, or for May to tell them campaigning can begin.

“We’ve been far too up our parliamentary arses,” admits one people’s vote strategist, promising that the battle will now move to street level, with another mass demonstration on the way. He admits that they’ve paid too much attention to Westminster, which is the weather, rather than building public opposition to Brexit, which is the climate.

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So start the campaign now. And as pro-Europeans make their case, they need to be mindful that economic catastrophism – even the sincere warnings of medicine shortages and actual corporate decisions to relocate jobs – is not cutting through. As Gary Younge wrote on these pages last week, remain needs to tell a story that goes beyond economics and speaks to national pride.

As it happens, such a narrative is there, ready to be told. It is the story of a nation that has always been plural, hybrid, forged of many tribes; of a confident people who have reached across the seas to touch their closest neighbours, and who have never feared any kind of international club or alliance, because they had the confidence, even the swagger, to know they did not just belong in such a grouping, they could help shape it.

Our ancestors would not recognise today’s Brexiters, so cowed and lacking in courage that they fear they can’t cut it at the top table, sitting as equals alongside France and Germany, but must hide instead in the corner, on their own. The nation whose earliest works are now on display at the British Library would see the European Union and know in an instant that its place was at the heart of it.

So we do have a story to tell. But let’s not wait for May’s permission to tell it. There are fewer than 50 days to avert what will be one of the gravest errors in our long history. There is not a moment to waste.

• Jonathan Freedland is a Guardian columnist