Should the United Kingdom remain in or leave the European Union? That's the question the British public will decide in a referendum on June 23. WSJ's Niki Blasina explains the debate and what's at stake.

EACH Saturday through the main shopping strip of Gibraltar on Spain’s southern most outcrop, a troupe of nationalist British volunteers march in 18th military Redcoats.

The weekly midday march honours the 1779 Great Siege in the war between Spain and England where at sunset the garrison sergeant with armed escort would lock the gates to the city, fife and drums warning “aliens” it was time to leave.

It is the sort of Britishness pomp typical of the Rock of Gibraltar that still boasts traditional bobbies, red phone boxes and bangers and mash on every street corner pub.

But come this Thursday it could be Spain that shuts the frontier gates of the city.

On this day the UK will vote on whether to stay part of the European Union or go its own way.

The referendum vote is final and irreversible with no second chance.

And at this stage a break away from the union is looking likely.

RULE, BRITANNIA

It’s uncharted waters, no country had every tried it before and the overall effect on the European Union could be spectacular. Prime Minister David Cameron has predicted many dire outcomes should the vote “Leave” be successful and in a microcosmic way has used Gibraltar as a case in point.

A report released in February warned “there would be no certainty” the borders to the British Rock would remain open if the Brexit (Britain exit) vote was successful, in much the same way as for the whole of the UK’s now open door access to Europe.

But there is a lot more at stake if Britain opts out which polls are now predicting including loss of automatic trade market access, security, prestige and global standing, isolationism and, if one believes the argument, the beginning of the end for the union of Europe. The stakes are high and in or out the nation will never be the same again.

So how is it Britain now stands on the precipice of what many European leaders have predicted could be the beginning of the end for the 28-nation EU bloc and why is it exactly some Australians can vote in such a decisive poll?

BRITONS NEVER SHALL BE SLAVES?

In 2013, Cameron vowed to hold a referendum on the UK’s future within the EU if he won re-election and could not gain more favourable arrangements for the UK within the bloc.

Critically Cameron wanted changes to allow Britain greater control of police and courts including power to expel criminals who relied on EU law to remain in Britain, restore greater power to national governments over the Brussels-led EU parliament, tougher immigration including restrictions on medical and social benefits for EU visitors solely coming to Britain for better health, welfare and child care and significant reductions to EU bureaucracy.

Cameron’s EU negotiations largely failed but the leader, now bound by his promise for a referendum and aggression from the right-wing of his own party, has since last year been campaigning for the UK to remain within the EU anyway, believing some minor concessions he had achieved could be built on overtime and it was better, safer and more predictable to remain within.

The call split his Tory party and the British people and today polls are still 50-50 on the simple Remain or Leave question.

GUARDIAN ANGELS SANG THIS STRAIN

Regardless, all sides have agreed come Thursday’s vote it would be binding, and if it’s an exit, immediately the mechanisms of the untested EU treaty Article 50 for a nation to leave the bloc would begin and within two years the UK would be totally out.

Just what that will mean has lead to reams of commentary, at times hysterical and emotive, from nothing will happen if the nation leaves to the nation would be overrun by millions of Turks and refugees and even the cost of the humble Magnum ice cream would increase. Oh yes and somewhere in the middle, the end of Western civilisation as we know it.

“Why is it so dangerous? Because no one can foresee what the long-term consequences would be,” European Council president Donald Tusk said this week. “As a historian I fear that Brexit could be the beginning of the destruction of not only the EU but also of western political civilisation in its entirety.”

NATIONS NOT SO BLEST AS THEE

On the basis of his dire prediction European shares dropped and the value of the pound continued its roller coaster ride amid predictions an exit could spark war, recession or both.

The complication for the Brexit vote is it comes on the greatest social upheaval in Europe since the Second World War, with the influx of refugees from Syria and elsewhere, the rise of ISIS and widespread terrorist attacks and threats, the rise of the Far Right across Europe, aggression from Russia over Ukraine and response by NATO this week to deploy four multinational battalions to the Baltic States and Poland to “send a clear signal” to Moscow.

THE DREAD AND ENVY OF THEM ALL

What is clear that left to the people to go ahead without a country’s usual checks and balances and oversight of a parliament to make such a critical decision, either vote will be bitter, permanently divide the nation and the government and in some quarters create the sort of social panic and uncertainty that months of hysteria debate has already generated.

But one thing won’t change in Gibraltar or the UK — a sense that any decision will be very British.

“I expect the vast majority of people on the Rock will be voting to remain in the European Union but trust me, centuries of sieges — military, economic and political — have never changed our minds about being British,” Gibraltar chief minister Fabian Picardo has declared.