Like the euro currency, the Schengen zone has failed its first test. Both were fair-weather schemes; both are being shredded in the high winds of political crisis.

Just as the credit crunch pitilessly exposed the weakness of Europe’s monetary union, condemning millions across the Mediterranean to poverty and emigration, so the surge in migration from Africa and the Middle East is wrecking the concept of a border-free EU.

The timing, for British Europhiles, could scarcely be worse.

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Migrants jump out of a lorry after being discovered by French police as they attempt to cross the Channel

In probably less than a year, Britain will vote in a referendum on whether to leave the EU.

Yesterday, David Cameron accepted the Electoral Commission’s advice that his preferred wording for the ballot paper was loaded and, to his credit, accepted its proposed version instead.

So, instead of voting ‘yes’ or ‘no’, we will be asked to vote to ‘remain’ or ‘leave’.

Frankly, events across the Channel are making a ‘leave’ vote more and more likely.

Every month’s trade figures show our exports to the rest of the world rising, but our exports to the EU falling as the euro crisis lingers.

For me, this has always been the strongest argument for independence.

But for many voters, there is also the question of border control – a question we see vividly played out on our screens as migrants riot from Greece to Hungary, wanting to be allowed to press further into Europe.

The Schengen Agreement abolishes internal borders and passport checks among its signatories.

Pictured: Migrants trying to to sneak onto trains in Calais, France. The Schengen Agreement abolishes internal borders and passport checks among its signatories

Once you are in, you can travel unhindered across all 26 members.

To those who want the EU to become a single country, the scrapping of what they call ‘internal’ borders is an essential part of the dream.

Last week, the president of the European Commission, Jean-Claude Juncker, called Schengen Europe’s ‘greatest achievement’ – a title he generally reserves, with even more derangement, for the euro.

Others are more realistic.

Germany’s chancellor Angela Merkel says that, unless there is a more equitable distribution of migrants among EU states, the Schengen zone might have to be scrapped.

She evidently hopes to push other nations into accepting more settlers, as Germany has done.

But she may find that her European partners prefer to call her bluff.

When Schengen was incorporated into EU law in 1995, the UK and Ireland had the sense to stand aside.

Being islands, they wanted to retain the protection of the Channel which, as Shakespeare puts it, serves us ‘in the office of a wall, or as a moat defensive to a house’.

That decision looks wiser by the day.

But do not think our Schengen opt-out means we are not affected.

Someone from outside the EU can land unlawfully in Italy or Greece then travel to Calais without being stopped.

Migrants make their way across a fence near near train tracks as they attempt to access the Channel Tunnel

People have taken this route for longer than we sometimes realise. Non-EU migrants have been encamped around Calais since 2001.

Although their numbers rise and fall, there is a constant through-flow, as some enter Britain and others take their places.

Why Britain? For several reasons: our economy is growing while the eurozone’s shrinks.

Migrants are more likely to speak English than any other EU language.

Deportation orders are fitfully served and rarely enforced. And, of course, people like to come to places where they already have friends and family.

The last Labour government admitted nearly 4million people, meaning that most countries now have emigre communities in Britain.

We don’t know how many people have made the crossing; we can count only those detained.

But we can say for sure that they are not at risk of persecution in France. A refugee is someone who wants to get out of a particular country, not get into a particular country.

Not that anyone should be blamed for seeking a better life.

I’ve spent the past five days in southern Italy, working alongside underage migrants as part of a social action project run by the Alliance of European Conservatives and Reformists.

I witnessed heartbreaking stories of why these boys had left their home countries, and of the horrors they had faced crossing the Sahara and Mediterranean.

I like to think that, in their position, I would be brave enough to attempt the same journey. Still, few of them qualify under the strict definition of a refugee.

There is a part of us, of course, that says: ‘Never mind the strict definition. If someone is running away from brutality and misery, and wants to work, let him stay.’

We don’t know how many people have made the crossing; we can count only those detained

But think through the implications. There are tens of millions, perhaps hundreds of millions, in Africa and the Middle East who would then make the journey.

No country could cope with numbers on that scale.

What, then, is the solution? First, Schengen needs to go. The Euro-fanatics have done enough damage in their pursuit of political integration.

The restoration of sovereign borders would mean the authorities of each country no longer had an incentive to wave migrants through, knowing they would become someone else’s responsibility.

Second, we need to follow Australia’s example. Faced with a surge of seaborne migration, it towed ships to an island and processed claims offshore.

Since then, there has not been a single death through drowning and far fewer people attempt the journey.

To emulate Australia, though, we would have to stop interpreting the 1951 Refugee Convention in a way that obliges us to allow every asylum-seeker to remain in the EU while their claim is assessed.

That charter was designed in a very different age. Its authors were understandably haunted by the memory of the Jewish refugees from Nazi Germany in the 1930s who had been turned away from safe countries.

They could not have dreamed of an era of cheap travel, when perhaps two billion people could theoretically claim to be victims of oppression.

The restoration of sovereign borders would mean the authorities of each country no longer had an incentive to wave migrants through, knowing they would become someone else’s responsibility.

To do what the Australians have done, the EU would have to abandon a great many accords, starting with its Charter of Fundamental Rights.

That, though, is unlikely to happen. So if Britain wants to control its borders, it will have to do so unilaterally, by leaving the EU.

Ministers are tying themselves in knots trying to avoid this conclusion.

But all the schemes they propose – tightening benefits, requiring evidence of a job – fail to address the huge, clunking fact that, as long as we are in the EU, we cannot control who settles here or in what numbers.

Eventually, our hand may be forced. With no let-up in the numbers reaching Italy across the Mediterranean, there will come a point when the Rome government, tired of carrying the burden for other EU states, gives identity documents to all those migrants who want to move on and settle in Germany or Britain or Sweden.

The only question is whether this moment comes before or after Britain’s referendum.

If it comes before, no force on Earth will persuade people to remain in.