Boris Johnson has accused Remainers of using the Irish border issue to try to "frustrate" Brexit as a fresh row brews over the terms of the UK's withdrawal.
The prominent Brexiteer said that concerns over a potential hard border were being used as an excuse to keep Britain "effectively in the single market, so we can't really leave the EU".
Brussels today published draft withdrawal treaty setting out plans for a Northern Ireland to remain aligned with Brussels if no other solution to avoid a physical barrier can be found.
Theresa May has insisted that another way forward can be found to resolve the issue.
But concerns were reignited after Mr Johnson sent a letter to the Prime Minister saying it was wrong to suggest there would be no border and the focus should be on ensuring it does not become "significantly" harder.
Today he insisted the Irish issue was being "used politically" as a tool to keep the UK within the customs union.
"I think the particular problems around the Irish border are being used to drive the whole Brexit argument and effectively to try to frustrate Brexit," he told reporters.
Speaker John Bercow has granted Labour an urgent question on the future Irish border arrangements.
Any suggestion that Ireland could diverge from Great Britain will also be a red line for the DUP party which is propping up Mrs May's government.
Meanwhile Brussels' chief negotiator, Michel Barnier said any final Brexit agreement must avoid a hard border in Ireland as he published the EU's draft plans.
"Nothing in here will be a surprise for those who have followed the negotiations" he said.
He stressed that plans for Northern Ireland to diverge from the UK was only the final "backstop solution" if no other agreement could be found.
Mr Barnier also warned the UK it must "pick up the pace" as it gets to grips with details of the upcoming split .
The European Commission's draft text is based on agreements made between Mrs May and Commission president Jean-Claude Juncker in December.
Mr Barnier said he would meet the DUP's Arlene Foster and Sinn Fein's Michelle O'Neill next week.
He also warned that there were still significant problems in agreeing the transition period sought by Theresa May.
And he said the EU still believed that the European Court of Justice "must play a role for the interpretation and implementation of the withdrawal agreement whenever that agreement refers to European law".
Former Brexit minister David Jones said the EU's proposal was "completely unacceptable" and amounted to an attempt to annex Northern Ireland, in comments to BBC Radio 4's Today programme.
"I think that it would be pretty catastrophic and I think that the European Union in actually proposing this is behaving wholly irresponsibly."
Meanwhile, Downing Street was swift to play down Mr Johnson's suggestion here could be a physical barrier on the Northern Ireland frontier.
A source said: "We agree the task isn't about no border, it's about no hard border."
Ireland's Foreign Minister Simon Coveney said the proposals to keep Northern Ireland in the customs union were needed to protect the Ireland peace process in case talks fail.
He added: "This is very much a default and would only apply should it prove necessary. This is about delivering on our shared objectives of protecting the Good Friday Agreement and the gains of the peace process, no less, no more."
Mrs May is due to set out the UK's approach to the next phase of talks in an announcement on Friday.
She is also facing fresh pressure from backs of a soft Brexit after the Labour party said it backed remaining in the customs union.
It is likely to table an amendment to the Government's Brexit legislation, in which Tory rebels could join the opposition to inflict a defeat.