Theresa May will use a speech in Belfast on Friday to rule out agreeing to any Border backstop based on the European Union’s proposal to keep Northern Ireland in the customs union and parts of the single market.

She will tell the European Union to “evolve” its stance on Brexit, warning that previous positions are “unworkable”.

Speaking at Belfast’s Waterfront Hall, she will say that the EU’s proposal is in breach of the Belfast Agreement, leaving the people of Northern Ireland without their own voice in trade negotiations and would be destabilising for their economy.

“The economic and constitutional dislocation of a formal ‘third country’ customs border within our own country is something I will never accept and I believe no British prime minister could ever accept. And as they made clear this week, it is not something the House of Commons will accept either,” she will say.

If UK Govt don’t support current EU wording on Backstop in draft Withdrawal Agreement, then obligation is on them to propose a viable and legally operable alternative wording that delivers same result: no border infrastructure. Clear UK commitments were made on this in Dec+March. — Simon Coveney (@simoncoveney) July 19, 2018

Tánaiste Simon Coveney said if the UK rejects the EU’s proposal, the UK needs to come up with an alternative.

“If UK Govt don’t support current EU wording on Backstop in draft Withdrawal Agreement, then obligation is on them to propose a viable and legally operable alternative wording that delivers same result: no border infrastructure. Clear UK commitments were made on this in Dec+March,” he tweeted.

MPs this week backed an amendment to a customs Bill, proposed by Brexiteer backbenchers and accepted by Ms May’s government, that would prevent Northern Ireland being part of a different customs regime to the rest of the UK.

Border checks

In Belfast, the prime minister will say that the notion of a hard border is unacceptable and almost inconceivable. But she will argue that her Chequers proposal would solve the Border issue by creating a free trade area for goods which would avoid the need for customs and regulatory checks at the Border.

“We can negotiate a new relationship with the EU that works in our mutual interest. One that honours the referendum result, gives us control of our money, our borders and our laws. One that sets us on course for a prosperous future, protecting jobs and boosting prosperity. One that safeguards our union and allows the whole UK to thrive in the years ahead. A brighter future for Northern Ireland – where we restore devolution and come together again as a community to serve the interests of the people,” she will say.

The Chequers proposal and the White Paper the British government published last week have divided Conservatives but the prime minister will say on Friday that they amount to a realistic proposal and that it is now for the EU to respond. “Not simply to fall back on to previous positions which have already been proven unworkable. But to evolve their position in kind. And, on that basis, I look forward to resuming constructive discussions,” she will say.