Ireland has dismissed British proposals for the Irish border after Brexit as unconvincing, a day after the EU chief negotiator said they amounted to a demand the bloc suspend its laws for Britain.
But British Foreign Secretary Boris Johnson, speaking at a meeting of EU foreign ministers in Tallinn, said a solution was "not beyond the wit of man".
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The border between the Irish Republic and the British province of Northern Ireland is open to free flow of goods, being an internal EU frontier.
But when Britain leaves the bloc, it will become subject to EU customs regulation.
Establishment of a physical border could revive security concerns, 20 years after a peace deal involving Dublin that ended a long civil conflict in Northern Ireland and led to the end of army and police checkpoints.
Britain has proposed an "invisible border" without border posts or immigration checks after Brexit but given no firm proposals how the customs frontier between Northern Ireland and the Republic would be monitored.
EU Brexit negotiator Michel Barnier said on Thursday that British proposals would undermine the bloc's single market.
He said Britain in effect wanted the EU to "suspend the application of its laws" as a test case for broader EU-British customs regulations.
Irish Foreign Minister Simon Coveney told Reuters on Friday: "The maintenance of an invisible border on the island of Ireland would be a lot easier if Britain were to remain in the customs union."
That is something Prime Minister Theresa May has said would not happen, though her cabinet is split on the issue.
The European Parliament's chief Brexit speaker, Guy Verhofstadt, dismissed Britain's plans for an "invisible border" as surreal.
But, asked if he was confident that Britain would get a deal with the EU, Mr Johnson said in Tallinn: "Absolutely, with rock-solid confidence."
Negotiations to extricate Britain from the EU have seen a slow start and Brussels has repeatedly warned time is running out to answer complex questions before Britain is due to leave in March 2019.