The Prime Minister put herself on another collision course with Brussels, telling MPs she would make her staunch opposition to a draft legal agreement “crystal clear” to EU leaders.
The stance came after Michel Barnier, the EU’s Brexit negotiator published the text which proposes a "common regulatory area" across Ireland, if other solutions to avoid a hard border fail.
It would create a customs border in the Irish Sea unless Ms May agrees to alignment across the UK – effectively keeping the entire UK in the EU customs union and single market, which she has ruled out.
During Prime Minister’s Questions, she was asked, by a Democratic Unionist Party MP, to “never agree to any borders between Northern Ireland and the rest of the United Kingdom”.
In reply, Ms May said the Government would work with the EU to get their December agreement “translated into legal form in the withdrawal agreement”.
But she warned: “The draft legal text the Commission has published would, if implemented, undermine the United Kingdom common market and threaten the constitutional integrity of the United Kingdom by creating a custom and regulatory border down the Irish Sea.”
The Prime Minister added: “No United Kingdom prime minister could ever agree to it. I will be making it crystal clear to President Juncker and others that we will never do so.
“We are committed to ensuring that we see no hard border between Northern Ireland and Ireland.”
However, significantly, Ms May noted that the December text – which she shook hands on – also “made clear that there should continue to be trade between Northern Ireland and the rest of the United Kingdom as there is today”.
Before Christmas, after the DUP vetoed the proposal for the customs border to be moved to the Irish Sea, the Government agreed to “full alignment” across the UK, as a “backstop” solution.
For two months, it has been pointed out that this was likely to require the UK to remain within the customs union and single market – but the controversy has been fudged.
In the Commons, Ms May gave her fiercest denunciation yet of customs union membership, saying it would “betray the vote of the people” and appearing to give herself no wriggle room.
Speaking afterwards, Keir Starmer, Labour’s Shadow Brexit Secretary, said: “Theresa May’s failure to offer any viable solution to the border in Northern Ireland has come back to haunt her.
“And yet, 20 months since the referendum the Government has yet to set out how it can achieve this without the UK remaining in a customs union with the EU.”
Ms May told Jeremy Corbyn she would reveal more details of her Brexit proposals in a major speech at the end of the week.
In Brussels, Mr Barnier made clear his text did not include a commitment to no customs border between Northern Ireland and mainland UK only because it would be seen as interfering in the country's domestic affairs.
And he promised to “delete” the backstop proposal for alignment across Ireland if a trade deal or a British technological solution – yet to be put forward - solved the border dilemma.
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