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What happens at the Irish border if there’s no Brexit deal?

At face value, it seems straightforward—the EU’s land frontier with Britain would need to be policed to uphold the rules of the single market. Yet, Irish Prime Minister Leo Varadkar seems to take a different view. Speaking in Brussels on Thursday, he told reporters there are no preparations to install checks, and has consistently said Ireland won’t build a border.

“The U.K. has said they won’t do it,” Varadkar said in an interview with broadcaster TV3 this month. “And I’ve made it very clear to other European prime ministers and presidents that’s something Ireland will never do.”

It’s a position that puzzles some in Brussels. Speaking privately, one senior European official suggested the Irish should take a more measured approach or risk embarrassment if talks collapsed. Speaking on condition of anonymity, the EU official said there while there would be sympathy for Ireland in the event of the U.K. crashing out of the bloc without a deal, controls would eventually have to be implemented.

The border conundrum—how to keep the frontier open after Northern Ireland leaves the bloc with the rest of the U.K. and the Republic of Ireland remains in the EU—is the main stumbling block to an orderly Brexit agreement. But if talks break down without a deal, the border also risks prompting political turmoil on the island of Ireland and tensions at the heart of the EU.

Ireland would be given time to organize a border, and some leaks and smuggling may initially be tolerated, according to the EU official. But ultimately a border would probably have to be erected, the official said.

Ireland’s 310-mile (500-kilometer) border running from near Derry in the north to Dundalk in the south would become the customs frontier for both the U.K. and EU—where officials from both sides would check goods and levy customs duties, Federico Fabbrini, director of the DCU Brexit Institute in Dublin, said in a report last month. The report was commissioned by the European Parliament’s Constitutional Affairs Committee.

France is among countries most concerned about protecting the single market and any failure to adequately police the frontier in the event of a no-deal Brexit would create major problems with the European Commission and other member states, officials said.

Even if a wide-ranging trade deal was reached with the U.K., border posts might still be needed to check the origin of goods and to make sure they meet the rules of the importing country, Fabbrini said.

It’s the strong view of the Irish government that this won’t happen, according to one official.

Why Ireland’s Border Is Brexit’s Intractable Puzzle: QuickTake

“There will be no hard border, no border infrastructure, and no related checks under controls,” Irish Foreign Minister Simon Coveney said in an interview with broadcaster RTE last week.

Ireland points to British Prime Minister Theresa May’s commitments in December and March that no border will be allowed to re-emerge after it leaves the bloc. The government fully expects the U.K. to deliver on these commitments, a foreign ministry spokesman said in response to questions.

As yet, though, the U.K. has yet to sign up to the legally operable backstop that Ireland wants to keep the border threat off the table. The British position is that no matter what happens, it will stand by the Good Friday peace deal, though it’s not clear how that would be used to keep the border completely open in the event of the U.K. crashing out.

Last week, Coveney said he had been reassured by European allies that Ireland wouldn’t be left “isolated.” The same day, on a visit to Dublin, European Commission President Jean-Claude Juncker vowed Ireland could “count on me.”

Fabbrini identifies one possible way to square the circle. If there’s no deal with Britain, the bloc could invoke a so-called frontier traffic exception to declare the whole territory of Northern Ireland as a border region to the EU, Fabbrini said, citing a never-before used clause linked to the General Agreement of Trade and Tariffs.

“Clearly this solution would not be problem free,” said Fabbrini. “Nevertheless, it should remain in the armory of the EU.