This week, we Britons face the biggest democratic decision of our lifetimes. The outcome of the referendum on Britain’s membership of the European Union will shape our politics, our economy, our society and our role in the world for decades to come. We must be clear about what we would be turning our back on were we to vote to leave.

Much of the case to remain in the EU has been framed in terms of the economic risks of Brexit. Membership of the world’s largest single market has played a critical part in Britain’s transformation from the economic malaise of the 1970s into the world’s fifth largest economy. No one can predict exactly what the costs of leaving that market would be, but there is little doubt that they would be significant. The Bank of England, the IMF, the World Trade Organisation, the OECD and the World Bank have all warned of the risks. Nine out of 10 of the 600 economists surveyed for this paper last month think Brexit would damage Britain’s growth prospects.

But the European Union was always much more than an economic project. It was an idealistic undertaking, born out of the desire to never again see the continent racked by war.

This is easily forgotten in an age where the idea of European nations warring against each other seems inconceivable. But our continent faces newer, global challenges: the risks of climate change; the mass movement of people fleeing conflict and abject poverty in Africa and the Middle East; the deadly consequences of microbial resistance; the question of how to hold to account corporate behemoths that trade across national boundaries. The need for a collective of countries to find ways of acting together has never been greater.

The Observer has always been proud of its internationalist, liberal worldview – one that looks out on to the world and is open to its cultural, social and political influences. One of the principal dividing lines in this referendum is whether we want to be open, not closed; expansive in thought and deed, not insular, and small.

Many campaigning to leave the EU view it as a way to wrest control from what they see as an elite, bureaucratic institution far removed from Britons’ lives. But they hark back to a world where sovereignty and control can sit neatly within the borders of the nation state. That idea fast becomes meaningless when the big challenges we face are not the atomised products of single societies, but of the frictions that inevitably arise among the 7.4 billion people who share the planet.

The EU is the world’s most successful example of international co-operation. That does not mean it is perfect: of course it is not. Twenty eight countries working together create bureaucratic inefficiencies, costs and frictions. Of course there are people with competing goals and agendas working in European institutions. Of course there may often be a lack of clarity about priorities: in recent years, the union has simultaneously pursued ambitions to deepen and broaden, at cost to both. Of course there have been moments of crisis, such as the refugee and migrant crisis, where the EU has not appeared up to the task. And attempts to establish a single European currency have provoked deep social and political problems.

Despite its many flaws, this paper believes the EU has, without question, been a force for good. It has succeeded in its aim of preventing another European conflict. It has established the world’s biggest single market while preventing a race to the bottom on employment rights. It has brought former satellite states of the USSR within the European fold, nurturing nascent market economies and democracies.

And Britain has been far from being a bystander, watching passively as the EU chalks up its successes. Prime ministers from Thatcher to Blair have played a leading role in the creation of the single market and eastwards expansion. Britain has shaped Europe for the better as much as Europe has shaped Britain.

Outside the EU, our role in the world would be diminished. This matters: not as a throwback to bombastic colonialism, but because, as a significant economic power and force for liberal democracy, Britain has global responsibilities. To turn our back on them would be a dereliction of our duty. Internationalism, co-operation and compromise are the tenets of modern diplomacy. Whether or not we choose to live by them will affect the stability of the world our children and grandchildren inherit.

The case for remaining, then, must not just be framed in the language of economics, but in the wider vocabulary of the world we wish to inhabit. Yet the inability of mainstream politicians to address the unequal consequences of globalisation, of which Britain’s EU membership is just one aspect, is one reason why the referendum debate has been so dominated by the issue of immigration.

Immigration has come to symbolise the problems of globalisation. While it has brought net benefits to the economy, it has affected jobs, services and housing in areas that have experienced large amounts of European migration in a short space of time. Moreover, immigration has become a focal point for people distressed by the loss of economic and cultural identity in areas that may have been barely touched by immigration, but which have never recovered from the loss of the industries that shaped their economies.

Politicians from mainstream parties have struggled to address these concerns in the last decade. They have swung between denying the existence of any problem, citing macro statistics, and adopting simplistic and, ultimately, patronising policies, such as a net migration target the government never had any hope of meeting.

In the course of this referendum campaign, something uglier has developed. While both sides stand accused of exaggeration, some politicians in the Leave camp have strayed beyond the realms of decency to stoke up fears about immigration with deliberately misleading campaign material, which borders on the xenophobic and racist. The Leave campaign have also fostered an anti-expert sentiment. In the face of increasingly complex global problems, this position is absurd – to wrestle with issues, among many others, such as climate, gene editing or food scarcity we will need the input, advice and support of experts.

The risks of Brexit go beyond the economic. One of the greatest dishonesties at the heart of the Leave campaign is the claim that many of those whose support it is attempting to attract would fare better in an isolated Britain shaped by the politics of the Brexiters. They wouldn’t. For people who have suffered the raw end of globalisation, there is little of promise in the worldview shared by many Leave politicians: less regulation, fewer employment rights, a greater embrace of the free market and further privatisation of public services.

A serious threat to the future of the United Kingdom also lurks in a vote for Brexit. It seems likely it would be followed by a second Scottish independence referendum.

Last, there are risks to the future of the European project. At a time when the case for a European Union is stronger than ever, Europe faces immensely difficult questions over the coming decade. There is a fundamental tension between deepening and broadening the union: the Observer has argued that given the limits of its popular legitimacy, and the growing threat to global stability from Putin’s Russia and the Middle East, the EU must now prioritise broadening over deepening. But this means it must find a way of making the eurozone sustainable without taking steps to further political integration beyond what its citizens would accept. It will likely have to revisit the workings of one of its fundamental principles, the free movement of people.

Europe is fragile and a British exit could prompt its unravelling. Since Churchill’s 1946 address to the University of Zurich, in which he implored France and Germany to build a European family with the full support of Britain, the UK has been at the table to work with our European allies in finding solutions to the big challenges of the age. In the past five years, we have started to step back from that role: British politicians used our opt-out to excuse ourselves from any attempt at a pan-European solution to the refugee crisis.

Leaving Europe while questions hang over its future direction would mark a fundamental shift for the worse in British foreign policy, for Europe and for Britain. Our future is profoundly affected by the fortunes of the continent of which we are a part. Leaving the EU does not change that – it simply means we will have far less influence over how our future pans out.

There is much the referendum result will not settle. The debate has illuminated a growing cleavage in modern Britain, with the winners of globalisation on one side, and the losers on the other. The campaign – and the result – will likely reveal profound divisions in Britain across class, age and geography. Whatever the result, whoever is left holding the reins of power, our leaders will have to find a way of reuniting the country after a divisive campaign and one that showed that economic and cultural stresses can easily loosen the binds that tie us together. As politicians attempt to close the fissures of this campaign once the result is declared, they should bear in mind the words of Jo Cox, brutally killed in the line of duty last week: “We are far more united and have far more in common with each other than things that divide us.”

It would be disingenuous to say that the choice that Britain faces this week is between a flawless, international union or global isolation; between economic nirvana or ruin; between security in perpetuity or the breakdown of order. No such choice exists.

Rather, it is the choice between going it alone or as part of a messy, imperfect collective, requiring co-operation and compromise. Between meeting the economic challenges all rich developed nations will face, as part of the world’s biggest single economic market, or outside of a trade grouping. Between confronting global insecurity as part of a bloc that shares our liberal democratic values or by ourselves.

Remaining in the EU will not magically eliminate the challenges Britain faces in the years to come. But if we choose to do so, it will keep Britain at the heart of reforming the European project so that the nations of Europe are together better equipped to face them. At its core, the European Union remains a practical expression of the belief that liberal democracies can achieve more acting in concert than they can alone. We must not turn our backs on that.