The European Parliament is to call for Northern Ireland to stay in the single market and customs union - a move that would "place an international border" between the province and the rest of the UK, a unionist MEP has warned.
However, MEPs have concluded it is the best solution to the problem of ensuring there is no physical border in Ireland.
It would mean continued free movement on the island of Ireland, with customs checks instead taking place at ports on the Irish Sea for visitors travelling between Great Britain and Northern Ireland.
The resolution, set to be voted on hours before Theresa May's make-or-break Tory conference speech, rubbishes Britain's existing proposals on the issue and is set to enrage DUP MPs propping up Mrs May's Government.
Both the UK and EU believe there should be no "hardening" of the Irish border but disagree on how to do this while the Republic remains inside the EU's borders and the UK leaves them.
It is understood that European Parliament chiefs believe shifting border posts to ports is the optimal solution. One source said the EU's physical border had to be somewhere and could not just have a gaping hole in it.
The resolution rubbishes Britain's proposals for a border based on spot-checks and says that the UK plan for a lack of physical infrastructure "presumes that the United Kingdom stays in the internal market and customs union or that Northern Ireland stays in some form in the internal market and customs union".
The PM has ruled out keeping the UK as a whole in the customs union or single market.
Ulster Unionist MEP Jim Nicholson said such an arrangement would deny British sovereignty here and would therefore breach the Belfast Agreement, which MEPs and EU leaders have already pledged to protect. Mr Nicholson said: "Keeping Northern Ireland in the customs union and single market with the rest of the UK outside, would place an international border between Northern Ireland and Great Britain and breach the national sovereignty of the UK over Northern Ireland.
"It would seriously harm our trading relationship with our most important market, a market in which we are at present fully integrated and which forms part of our own country."
TUV leader Jim Allister, a former MEP, tweeted his criticism. "These Euro fanatics need to be told that constitutionally the UK, not EU, determines our borders. Carving up UK is not on," he said.
However, Sinn Fein MEP Martina Anderson said the resolution "reinforces the right of people living in the north to Irish and therefore EU citizenship".
"This resolution is significant as it recognises our unique position and the need for us remain in the single market and customs union," she said.
Yesterday in Brussels, the European Commission's chief Brexit negotiator Michel Barnier said the resolution was "important", mentioning it alongside next month's European Council meeting as one of the hurdles the UK would have to clear before it could progress to the next stage of talks.
The resolution says the European Parliament "believes that it is the responsibility of the UK Government to provide a unique, effective and workable solution that prevents a "hardening' of the border, ensures full compliance with the Good Friday Agreement in all its parts, is in line with EU law and fully ensures the integrity of the internal market and customs union".
A Government spokesperson said: "We recognise and respect the vital role the European Parliament will play in this process.
"However, this is a draft document and was issued before negotiations this week were completed. Therefore it does not take into account the further progress made this week.
"We and the EU have committed to protecting the Belfast Agreement and the Common Travel Area and agree that we will not accept any physical infrastructure at the border.
"We recognise that the solutions to the unique circumstances in Northern Ireland must respect the integrity of the EU single market and customs union. But they must also respect the integrity of the UK."