Britain faces its biggest peacetime crisis since 1945. Leaving the EU on the slightest of referendum majorities, after a campaign in which the leave side was never constitutionally compelled to agree what leave meant, is already plunging Britain into a constitutional, political and legal quagmire. Prolonged economic stagnation, perhaps depression, seem inevitable. A liberal, tolerant, outward-looking country is being transmuted into an illiberal, intolerant, inward-looking one. This is as fundamental as it gets. Battle is being joined for our soul, yet many of those who see all they hold dear being ripped away from them are strangely mute. Nigel Farage was right when he said it was a revolution “without a shot being fired”.

It is time to return fire. But the political truth is that no success is possible without the full-throated support of the Labour party, strikingly absent from the unfolding trauma. That must change – both in the interests of the country and the people it aims to serve.

It is the great unravelling. The painful economic uncertainties of unwinding 43 years of EU membership must surely trigger an investment strike and consequent economic stasis. Britain has a $1tn stock of foreign direct investment – equalled only by China and the US – involving some 500 multinationals. Their regional or global headquarters are in Britain because of our participation in the European single market. Leaving it, as the Japanese government and the US Chamber of Commerce have both unequivocally warned, will seriously affect investment. The impact will cascade down into every nook and cranny of our economy.

Hopes of an export boom are overdone. Our export sector is uniquely weak. Britain’s share of world markets has been falling for decades, largely because of unaddressed systemic economic weaknesses. The response to the devaluation after the financial crisis was paltry. It promises to be paltry again. One prominent economic forecaster – the National Institute of Economic and Social Research – foresees a drop of up to 60% in our service exports as a result of Brexit. Exports of goods will have to boom just to offset that effect. They can’t and won’t.

British society is in no great shape to handle the consequences. Inequalities of income and wealth between generations, regions and classes are unedifyingly high. But addressing these inequalities requires financial resources – which will be reduced by Brexit. The public sector deficit in the current financial year is already £14bn higher than projected. The Resolution Foundation warns that just on a modest downward revision of growth, the deficit will widen by £23bn by 2020-21 – a cumulative shortfall of £84bn. It is almost certainly an understatement.

As serious is the impact on our culture and national conversation. Although leavers talk the language of embracing the globe, they simultaneously talk of controlling borders to keep the foreign “other” out. Mounting anti-foreigner sentiment is morphing into a generalised language of reaction; bigots of all descriptions – racists and misogynists alike – have a new licence to “speak the unspeakable”, as they would describe it. This is not the backdrop for a major mobilisation and programme of enlightened reform. It is the backdrop for retreat, closure and stagnation.

So what to do? The first thing is to get as passionate about what is happening as leavers were in making their case. Britain must not leave the EU. It is not just that leaving is an act of wilful economic self-harm – it is also abandoning the continent’s most noble project. This will be attacked by leavers as defying the democratic will. But democracy is about constant argument and deliberation, not permanently deifying one vote at one moment in time.

But to have traction, the argument has to be coupled with a political challenge. It must be made by a political party serious about winning political power, and capable of doing it. The electorate must be convinced that their darkening circumstances can be feasibly and realistically changed. A second referendum in this context is no more than an enabling building block in a larger programme of improvement.

There are many elements in such a programme. The case needs to be made for a repurposing of British capitalism, housed in a reimagined ownership, financial and innovation system that can seize the opportunities opened up by new technologies. The labour market must be recast to challenge the gig economy with trade unions that enfranchise ordinary people. Risk, which haunts so many lives, must be minimised by a renewed, cradle-to-grave, flexible and skills-based welfare system. The public realm must be reconceived with a sufficiently broad tax base to foster public initiative and enterprise. And Britain must reaffirm EU membership, which is the bridge to international openness and fundamental to our prosperity.

These are the propositions around which the centre and left – and the best of the conservative tradition – can unite. They are what the overwhelming majority of British people want, if only there were a party capable of making the arguments and putting together the coalition of interests to propel it to power to implement them.

Millions are aching to contest what is happening. We need a transformative public conversation – and a willingness to argue our heads off for tolerance, fairness and openness, as well as for membership of pan-European institutions and the markets on our continent.

This is above all a political project. It can encompass social movements, but cannot be delivered by them. This is why a group of centre-left Labour MPs has launched a new movement to make the case. I support their efforts because Britain stands on the verge of a great unravelling with untold consequences for its economy, society, place in the world, and its people’s souls. The standard must be raised: fire must be returned. We need to make the case for a reimagined Britain and its membership of the EU. We say not what we are against, but what we stand for. We want our country back. And we want it now.

This article is part of a speech delivered at the launch of the Labour Tribune MPs Group on Wednesday.