The British government has vowed repeatedly to end the free movement of people from the European Union when the U.K. leaves the bloc in 2019. But on Wednesday it acknowledged that, in one area of the country, it won't.
Britain said there must be no border posts or electronic checks between Northern Ireland and the Irish republic after Brexit, and it committed itself to maintaining the longstanding, border-free Common Travel Area covering the U.K. and Ireland.
"There should be no physical border infrastructure of any kind on either side of the border between Northern Ireland and Ireland," Conservative British Prime Minister Theresa May said.
That means free movement across the border for British, Irish — and EU — citizens. After Britain leaves the bloc, EU nationals will be able to move without checks from Ireland to Northern Ireland, and onto other parts of the U.K.
Free movement among member states is a key EU principle, and has seen hundreds of thousands of people move to Britain and get jobs there since the bloc expanded into eastern Europe more than a decade ago.
Many Britons who voted last year to leave the EU cited a desire to regain control of immigration as a key reason.
In a paper outlining proposals for the Northern Ireland-Ireland border after Brexit, the British government insisted it will be able to control who can settle in the U.K. through work permits and other measures.
It said "immigration controls are not, and never have been, solely about the ability to prevent and control entry at the U.K.'s physical border." Control of access to the labor market and social welfare are also "an integral part" of the immigration system, the paper added.
Northern Ireland is an especially thorny issue in Brexit talks, because it has the U.K.'s only land border with the EU — and because an open border has helped build the economic prosperity that underpins the peace process in Northern Ireland.
Since the 1998 Good Friday peace accord, British military checkpoints along the Ireland-Northern Ireland border have been dismantled, rendering it all but invisible. Thousands of people cross the 300-mile (500-kilometer) border every day.
Britain said it was determined that "nothing agreed as part of the U.K.'s exit in any way undermines" the Northern Ireland peace agreement.
The government's Department for Exiting the European Union acknowledged that "unprecedented" solutions would be needed to preserve the peace process and maintain the benefits of an open border after Britain leaves the EU, its single market in goods and services and its tariff-free customs union.
It suggested a future "customs partnership" between Britain and the EU could eliminate the need for checks on goods crossing the border.
For agricultural and food products, Britain said one option could be "regulatory equivalence," where the U.K. and EU agree to maintain the same standards. But it's unclear what that would mean for Britain's ability to trade with countries that do not always meet EU standards, such as the United States.
The Northern Ireland proposals came in a series of papers covering aspects of Brexit negotiations, which are due to resume in Brussels at the end of this month.
Irish Foreign Affairs Minister Simon Coveney said the document "brings some clarity and is certainly helpful to move this process forward." But, he said, "there are still significant questions that are unanswered."
European Commission spokeswoman Annika Breidthardt said Britain's position papers — which come after allegations from EU officials that the U.K. is underprepared for the EU divorce negotiations — are "a positive step."
"The clock is ticking and this will allow us to make progress," she said.
Lorne Cook in Brussels contributed to this story.