Arlene Foster, the DUP leader, has said news of a potential agreement between the UK and the EU on the issue of the Irish border came as a "big shock" to her party, which is propping up Theresa May in Parliament.
A potential agreement between London and Brussels that would have seen Northern Ireland retain "regulatory alignment" with the EU after Brexit fell apart on Monday after the DUP refused to give its backing to the proposal.
Ms May is reliant on the party's support after losing her parliamentary majority in June's general election.
Ms Foster said she had been asking for information on the Government's plans for the Irish border for five weeks but had only received news of the proposed arrangement on Monday morning.
She claimed British negotiators told her the Irish Government had insisted her party was not given advance notice of the text of the agreement.
"We are told the Irish Government prevented it coming to us," she told RTE.
"We hadn't seen any text despite asking for the text for nearly five weeks now.
"The text only came through to us late [on Monday] morning and obviously once we saw the text, we knew it wasn't going to be acceptable."
Ms Foster said the DUP considered "a situation where Northern Ireland is different to the rest of the UK" to be a "red line".
Speaking after negotiations in Brussels, Irish premier Leo Varadkar said a deal had been agreed between the UK, the EU Commission and the Republic of Ireland but was thrown into chaos at the eleventh hour when the DUP made clear it would not support it in Parliament.
Ms Foster said she had a "very open" telephone conversation with Ms May about why her party could not back the plan, adding that she told the Prime Minister that the situation "could have been dealt with differently".
The issue of how the Irish border is managed after Brexit has become a major sticking point during negotiations with the EU. UK ministers are desperate to resolve the issue in time for the European Council summit on 14 and 15 December, where EU leaders will decide whether talks have been positive enough to progress to the next phase, which relates to a potential trade deal.
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