Brexiters are courting a protest vote. Fed up with Brussels? Worried about immigration? Then vote Leave. In real life the June 23 referendum presents a choice — the most important the British have faced during the postwar period. The status quo has to be measured against the alternative. Is throwing up the barricades really an intelligent option? We will know soon enough which question the voters choose to answer.

Most of the time such confusions scarcely matter. The great thing about democracy is that citizens get a chance to change their minds. If a bunch of politicians renege on their promises they can be turned out next time around. The Brexit referendum is different: its consequences are asymmetric. If Britain opts for Remain it could well think again in, say, a decade or two. A Leave vote would be forever and would set out the path to the break-up of the UK. It is easy to see where the balance of risk lies — and why the Leavers prefer boorish anti-intellectualism to rational debate about what Out would look like.

One puzzle for people peering in from beyond Britain’s shores is why now? True, the EU is not in the best of shape but it has weathered the multiple stresses of recent years. In truth, the referendum’s timing has nothing at all to do with substance. It is all about party management: David Cameron thought a referendum would keep the Conservatives in one piece. Not one of the prime minister’s smartest calls.

The consequences of an Out vote would reach well beyond the UK, even if one assumes that central banks have learnt how to deal with shocks on financial markets. Donald Tusk, the Polish president of the European Council, was probably exaggerating when he suggested it could mark a beginning to the end of western civilisation. But it would be a shattering blow to the rest of the EU and to the standing of what broadly constitutes the west. For Britain the short-term costs would be high. The long-term consequences, political and economic, would be profound.

There is nothing complicated or abstract about the case for European engagement: it rests on three pillars: national prosperity, security and attachment to values that many Brits would claim as their own — liberty, democracy and the rule of law. This in an age when the west’s interests and values are under rising challenge from autocrats across the globe.

The dirty little secret of EU membership is that it has been an economic success story. Britain joined in 1973 as the sick man of Europe. In the subsequent 43 years it has flourished. National output has risen faster than that of Germany, France and Italy. Per capita gross domestic product has increased by an average of 1.8 per cent annually, against 1.7 per cent in Germany, 1.4 per cent in France and 1.3 per cent in Italy.

Business has gained from sharper competition, ready access to the single market and big inflows of long-term investment. As the OECD records, Britain has also held on to the light-touch labour regulation that gives it an edge over its European partners. So much for the charge that Brussels has held Britain back.

In depth UK’s EU referendum © Getty News, comment and analysis on the referendum to decide whether Britain will leave the EU.

Whisper it ever so quietly, but the economy has also benefited from the arrival of ambitious hard-working young people from the rest of the continent. The failure on immigration has been one of successive governments at Westminster that have denied cities and communities the resources needed to adjust to large flows of migrants.

Postwar prosperity has depended on security. The US commitment to act as the guarantor of peace on the European continent has been, and remains, in­extricably bound up with integration within the EU. Nato has primacy in military affairs, but the political and foreign policy cohesion provided by the union is equally important to Washington. It was the EU rather than Nato that imposed sanctions when Russia marched into Ukraine, and the EU that provided the opening for successful nuclear negotiations with Iran.

The revanchism of Russian president Vladimir Putin, nuclear proliferation, the terrorism of Isis, uncontrolled migration from the Maghreb and Sahel, inter­national criminal networks and climate change — all these present threats requiring a collective response. By weakening Europe, Brexit would

make it harder to address them; by weakening Britain they would diminish its capacity to shape the responses.

The pre-Enlightenment mindset of the Out campaign has little time for

values. Human rights are cast as a

European plot. Never mind that Britain was a moving force in creating the

European Court of Human Rights. Migrants are demonised as criminals and scroungers. Ironically, the self-styled upholders of parliamentary sovereignty want to overturn, well, parliamentary sovereignty: two-thirds of MPs oppose Brexit.

There is hard-headed interest here alongside the moral dimension. The open international system on which security and prosperity rest is in

turn rooted in the rule of law. Britain, with global interests, is particularly reliant on a rules-based system. Throw away the values underpinning the

rules and you invite a return to Hobbesian chaos.

None of this, one supposes, troubles Brexiters. Theirs is the mantra of populists through the ages: go on, give the establishment a kick. You can see the appeal. But look at the cost.

philip.stephens@ft.com