This is the year to stop Brexit. There will not be another chance. If by the end of this year the British parliament has approved a transition agreement with the 27 other members of the EU, including the framework for a future trading relationship, Britain will exit.

This will constitute the most gratuitous and consequential act of national self-harm in Britain’s postwar history. It will also do significant long-term damage to the larger project of bringing the countries of Europe together to defend our shared values and way of life in a world increasingly shaped by China, climate change and job-threatening automation. Stopping Brexit is mainly a task for Brits, but it is also, in smaller part, a challenge for fellow Europeans who recognise this truth.

For us British Europeans, the uphill task is to persuade more Brexit voters, and with them the currently temporising Labour party, that what are still mainly modest or intangible losses are harbingers of much worse to come. In the opinion polls, only a small percentage of those who voted for Brexit have thus far changed their minds, but a growing number think that Britain will get a bad deal.

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The Financial Times recently calculated that Brexit is already costing the country almost £350m a week, rather than making the same sum available for the NHS, as the Brexit battle bus deceptively promised. Experts predict a further hit on GDP growth in the coming years. They also reckon that the effects of Brexit will be worst for some of its strongest supporters: less skilled workers and those living in the north of England. With Scotland wishing to remain in the EU, and Northern Ireland wanting to keep open borders, the British union is under entirely predictable strain. There is growing anecdotal evidence that Britain’s voice already counts for less in the world. Brexit is taking the Great out of Britain.

Yet there are formidable obstacles to swinging public opinion within a few months. FT calculations and expert predictions are the last thing to persuade populist voters. As one lady in Newcastle memorably rebuked the academic Anand Menon: “That’s your bloody GDP, not mine!”

Former politicians like Tony Blair, unelected Lords such as Andrew Adonis, elevated diplomats and cosmopolitan business people, not to mention Guardian columnists and Oxford professors like me, are not best placed to sway the lady in Newcastle. The predominantly Eurosceptic press pumps out a relentless propaganda of success. The Daily Mail is always at hand to denounce “enemies of the people”. But we shall not be intimidated. As the Brexit secretary, David Davis, wrote before the referendum: “If a democracy cannot change its mind, it ceases to be a democracy.”

A further difficulty in making our case is this: what parliament votes on this autumn will not be the final shape of Brexit. It will be the terms of the transition period, plus some “framework” for the future relationship. By then it should be clear to everyone that Theresa May’s government can’t have its hoped-for, uniquely generous bespoke deal – but a combination of negotiators’ fudge and mendacious rhetoric can make the government’s current second option, deceptively labelled “Canada”, sound attractive to British ears.

Facebook Twitter Pinterest ‘Former politicians like Tony Blair, unelected Lords such as Andrew Adonis (above), elevated diplomats and cosmopolitan business people are not best placed to change the public’s mind on Brexit.’ Photograph: Oli Scarff/Getty Images

Actually, Britain is in a completely different position to Canada, and a Canada-type deal would therefore be much worse for us – since so much more of our trade is with our European neighbours, and the Canadian agreement does not cover the services which make up 80% of Britain’s output. But slippery Brexit orators like Michael Gove will cry: “Don’t listen to those so-called experts and sourpuss remoaners: the future is Canada, the future is bright!” There’s a real danger that part of the British public will again be Goved.

Steep though it is, this is the hill up which we must climb. We need all the help we can get, and it would be nice if even a little support came from fellow Europeans across the Channel. I’ve been surprised, and frankly somewhat pained, by the speed with which most continental Europeans have accepted Brexit, treating it as something that has already happened.

Among themselves, they hardly talk about it, and even then only with a pitying laugh. Indeed, there were moments in 2017 when it seemed the one thing on which all the 27 other members of the EU could agree was being tough on Britain. Now, admittedly, our fellow Europeans have enough problems of their own, from the eurozone and refugee issues to Vladimir Putin and central European populists. Yet I fear they underestimate the long-term damage that the departure of a major member state will do to the whole European project.

Polling by YouGov suggests that more than a third of French respondents would prefer Britain to leave the EU, against less than a third who would like it to stay. But in Denmark, Sweden and Finland an absolute majority of those asked say they would prefer Britain to stick with them. Some 60% of Germans say they would be relieved, pleased or delighted if Britain decided to remain in the EU after all. Well, if you really think that, why not say it loud and clear, so British voters can hear it from you?

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It has been suggested that Emmanuel Macron, as the go-to European politician of 2018, should lead this continental chorus. Recalling the 700-year history of Anglo-French rivalry, I’m tempted to suggest that it might be more effective to let the English know that the French really want them to leave. But joking apart, every continental European voice will be welcome. And, for that matter, Canadian, Indian, American or Brazilian voices, reflecting on the wider damage to liberal international order.

To be clear: I have no illusions about the probability of stopping Brexit. I’m not surprised that so many British politicians, practising “the art of the possible”, are now advocating halfway houses such as continued membership of the customs union (the only way to ensure a genuinely open border across the island of Ireland) and/or the single market (“Norway” as opposed to “Canada”), although they know perfectly well that the only good option is to remain.

It may be that in the end we are compelled to settle for some worse alternative, such as being a Norway-like rule-taker rather than rule-maker. For the economic wellbeing of Britain, and particularly for that of poorer groups and regions who voted for Brexit, this would still be a less bad outcome than setting off for the fairytale never-never land of buccaneering neo-Elizabethan independence – a Trump-like mirage of “Global Britain” that would rapidly collapse into a putrid reality of being Greater Cyprus with missiles. Any Labour MP who does not recognise that will be betraying his or her hard-pressed voters.

Yet so many improbable things have happened in world politics over the last two years that perhaps we should redefine politics as the art of the improbable. Without illusions, let us fight for the only good outcome, for Europe as well as Britain. Expect the worst, work for the best.

• Timothy Garton Ash is a Guardian columnist