Boris Johnson's Brexit proposals will lead to a no-deal if significant changes are not made, the Tánaiste has warned.
Simon Coveney has made it clear that the proposals put forward by the British prime minister are "not the basis for a final deal or an agreement".
Speaking in the Dáil, Mr Coveney said there are some positives in the four-page letter sent to EU Commission president Jean-Claude Junker, but he added: "If that is the final proposal then there will be no deal."
Mr Coveney welcomed the fact that there are some positives contained in the last-ditch Brexit plans, but also highlighted two significant problems which he said were very clear from the outset.
He said Brexit is now "the issue of the day, despite storms and rugby matches and so on, these are really serious issues".
Citing difficulties around customs checks and a clause to give the Northern Ireland Assembly a sign-off on regularity alignment, he warned: "This isn't about cutting some deal at the last minute".
Mr Johnson suggested that an all-island regulatory zone, covering all goods including agrifood, would depend on the consent of the Northern Ireland Executive and Assembly.
As part of this the institutions would be asked to endorse the arrangements every four years.
Questioned by Fianna Fáil deputy leader Dara Calleary, who described the Assembly clause as "very vague" Mr Coveney said: "We cannot support any proposals that suggest that one party or indeed a minority in Northern Ireland could make the decisions for the majority.
"That would not be consistent with the Good Friday Agreement and it would not be something that we could support as part of any final deal."
He said talks would now be held in Brussels between UK and EU negotiating teams.
The Tánaiste said the proposals would form the basis of further discussion to get both sides into a "landing zone" for an agreement, but he added that Europe would be "calm, respectful, but very firm".
Sinn Féin TD Martin Kenny said the plans tabled by Mr Johnson are "totally unworkable".