A “flexible” Brexit, under which those parts of the United Kingdom that voted in June’s referendum to stay in the European Union would remain members of the single market, may be the answer to many political headaches, but it’s almost impossible to pull off.

Such a scheme might work for Gibraltar and Northern Ireland, which share land borders with the EU but not with Great Britain.

But it’s difficult to imagine how this could work for Scotland. Would continental goods that were exported tariff-free to Scotland be taxed at the English border? What about Europeans visiting Edinburgh or Glasgow without a visa? Would they need to go through customs if they wanted to see other parts of the United Kingdom?

Scotland’s first minister, Nicola Sturgeon, has nevertheless called for flexibility, “particularly for those parts of the United Kingdom that voted decisively to remain in Europe.”

62 percent of Scots voted to stay in the EU this summer against 47 percent of the English.

The majority of voters in Gibraltar, London and Northern Ireland opted to remain as well.

“No opt-out”

Theresa May, the prime minister, has promised her Conservative Party there will be “no opt-out from Brexit.”

“Because we voted in the referendum as one United Kingdom, we will negotiate as one United Kingdom and we will leave the European Union as one United Kingdom,” she said earlier this month.

An aide similarly told the Financial Times there would be “no exemptions” for parts of the country.

One border must close

Unless the EU and Britain as a whole negotiate a Norway-style arrangement, under which the island nation would remain in the single market, Scotland could probably not keep its border with the EU open without closing its border with England.

Which is precisely what Sturgeon is after when she says a second independence referendum for Scotland may be in the cards.

So long as May insists on curtailing EU immigration and the EU maintains that its “four freedoms” — free movement of capital, goods, services and people — cannot be separated, a British departure from the single market is inevitable. When that happens, expect Scotland to reevaluate which open border it prizes the most.