In a speech in Belfast on Friday she is expected to brand the bloc’s calls for regulatory alignment north and south of the border as a “backstop” solution in the event of no deal as “unworkable”, and repeat her assertion that a border down the Irish Sea is unacceptable to any British prime minister.

“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,” she will say.

May will tell an audience of business leaders and politicians that the EU proposal is in breach of the Belfast Agreement because it would create a barrier between Northern Ireland and Great Britain and leave the people of Northern Ireland “without their own voice” in trade negotiations. “It is not something the House of Commons will accept,” she is due to say.

The EU’s other 27 states will have a chance to examine and respond to the white paper when its General Council of ministers meets in Brussels on Friday morning. They will also receive an update on negotiations from the European Commission’s chief negotiator, Michel Barnier.

May’s decision to push back so strongly against the EU and Ireland’s demands for a backstop will fuel fears in Dublin that she is backsliding on the joint agreement in December to secure insurance in the event of no deal.

Her opposition to a border in the Irish sea was cemented on Monday when a last- minute amendment to the customs bill, tabled by the Labour MP Kate Hoey, was nodded through making it illegal to have a barrier between Northern Ireland and Great Britain.

May is on a two-day visit to Ireland – her first to the Irish border. She spent two hours meeting business leaders at a pottery factory in the village of Belleek on the Fermanagh and Donegal borders, but did not take questions from reporters.

Before leaving the factory, May met local woman Delma Käthner, who told her she was “bionic”. “She’s coped with so much,” said Käthner. “She has a terrible job. Just look at the way her shoulders are hunched. She has the whole weight of Brexit on her.”

May is expected to say in her speech that it is “now for the EU to respond” after the publication of the white paper, and to show the same flexibility as her government “not simply to fall back on to previous positions which have already been proven unworkable, but to evolve their position in kind.”

The PM will also say that the EU’s proposals for a backstop would destabilise the economy in Northern Ireland.

Her remarks come hours after the Confederation of British Industry said that new economic data showed that Northern Ireland was edging towards recessionary territory, with the uncertainty over Brexit partly to blame.

Ireland’s taoiseach, Leo Varadkar, has expressed concern that the turmoil in the House of Commons suggests a withdrawal agreement would never be supported in Westminster, whatever the outcome of Brexit negotiations.

On Wednesday night Varadkar said Ireland was looking to hire about 1,000 officials for customs, veterinary and export checks to cope with a no deal.

May, who is relying on the Brexit-supporting Democratic Unionist party to maintain her majority in the House, will also use her speech to reassert her commitment to the union of Great Britain and Northern Ireland.

“For all of us who care about our country, for all of us who want this union of nations to thrive, that duty [to achieve a deal for everyone] goes to the heart of what it means to be a United Kingdom and what it means to be a government,” she will say. “Our job is not to deal with Brexit in theory, but to make a success of it in practice for all of our people.”

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