The Democratic Unionist Party only received draft proposals on the Irish border from the Government on Monday morning before deciding they were "clearly unacceptable" and scuppering agreement in Brexit negotiations.
DUP Westminster leader Nigel Dodds blamed the Irish Government and European Union for the delay in seeing plans the party later rejected, because they would have shifted Northern Ireland's customs border to the Irish Sea.
The move by the DUP dashed Theresa May's hopes of securing agreement on the terms of Britain's EU withdrawal, which included the plan to avoid a hard Irish border.
The Prime Minister had to break off from talks with European Commission president Jean-Claude Juncker on Monday for an urgent call with DUP leader Arlene Foster, after she dramatically declared her party's implacable opposition to the plans.
At a Westminster press conference, Mr Dodds revealed that the DUP did not receive the proposals until just before Mrs May's lunch with Mr Juncker.
He said: "Despite several briefings over the course of the last few weeks, we only received written text late yesterday morning.
"We understand this was due in part to delays caused by the Irish Government and the EU negotiating team.
"Upon immediate receipt of that text we indicated to senior Government representatives that it was clearly unacceptable in its current form."
Mr Dodds said the DUP would "work for as long as it's necessary" until the issue was resolved because it is of such "vital importance" to the UK.
He also appeared to suggest the Irish government's "aggressive stance" on avoiding a hard border could scupper negotiations and lead to Britain leaving with no deal, and that hard border may therefore be erected anyway.
"The Prime Minister has said that there will be no border in the Irish Sea, she has made it clear that the UK is leaving the European Union as a whole and that the territorial and economic integrity of the United Kingdom will be protected," he said.
"So we want to see a sensible Brexit, and we will continue to work through the detail of all these issues with the Government today and in the coming days.
"These are issues that are of such vital importance to our nation as a whole that we must work for as long as it's necessary to ensure that they are got right.
"So the DUP does stand strong for the union and we also issue a warning today to the Dublin government - that by continuing its aggressive stance they are in danger of delivering for themselves the very outcomes that they said they want to avoid.
"So now more than ever it's clear that we took the correct view in encouraging people throughout the United Kingdom to vote to leave the European Union."
Mr Dodds stressed the DUP do not want to see a "no deal" Brexit.
"We don't want to see the talks fail and we don't want to see an outcome where there's no deal. We want to see a sensible Brexit and we will work through on the basis of the clear red lines we have set down, which are as we understand it the red lines of the Government as well."