Brussels/London – Brussels and London will begin working on a "backstop" option that would in effect keep Northern Ireland bound by key aspects of the European Union's single market and customs union, EU chief Brexit negotiator Michel Barnier says.

Barnier said that, in the absence of other proposals out of London, Britain could only avoid a hard border between the Republic of Ireland and Northern Ireland, which is part of the United Kingdom, by maintaining "full regulatory alignment" across the border.

"There must be no ambiguity"

Britain agreed in December that it would seek to avoid a hard border "through the overall EU-UK relationship," adding that, if this proved impossible, it would "propose specific solutions to address the unique circumstances of the island of Ireland."

Both sides agreed at the time that they would seek "full alignment" of post-Brexit regulations across the border in the absence of other solutions, with Prime Minister Theresa May adding later that this third solution was a "fallback option of last resort."

In the absence of other solutions, "it is our responsibility to include the third option in the text of the withdrawal agreement to guarantee there will be no hard border, whatever the circumstances," Barnier said Friday, following a week of Brexit negotiations.

"We must now start legally defining how this scenario would work in operational terms. There must be no ambiguity here," he said, adding that Britain had agreed to hold these talks, "provided we discuss the other two options in parallel."

Crucial issues

By October, when all aspects of Britain's departure from the EU are to be finalized, it must be clear how the Irish border issue will be settled, Barnier noted. Speaking earlier on Friday, Ireland's Europe Minister Helen McEntee urged May to clarify her plans for the post-Brexit Irish border, saying a solution to the crucial issue appeared "more and more difficult."

Ireland continues to believe that the best solution "would be for the UK to stay within the [EU] single market and customs union, or a form of customs union," McEntee told BBC Radio 4's Today programme.

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May ruled out Britain remaining in the EU customs union earlier this week, and has consistently rejected the idea of continued membership of the single market. "It seems more and more difficult the more proposals or options you take off the table," McEntee said.

Ireland wants clarity

"So we need the UK government, we need Theresa May and her team, to start looking at and to start, I suppose, letting us know what the proposals are ... and what their options are."

Discussing the possibility of "regulatory alignment," McEntee said the British government had "always been very clear... that they felt they would not need this backstop position or this backslide, that they would be able to work with the EU to create this close relationship." "So what we need to see now is how is that going to happen," she said.

The United Kingdom, which includes Britain and Northern Ireland, voted by a majority of 52 per cent to leave the EU in the Brexit referendum in June 2016.

In Northern Ireland, 56 per cent of voters opted to remain in the EU.