Theresa May pulled out at the last minute from a deal to break the Brexit logjam after meeting fierce resistance from the Democratic Unionist Party, Ireland's prime minister has claimed.
Crunch talks between Mrs May and European Commission president Jean-Claude Juncker ended in Brussels without a deal after the DUP - which props up the minority Conservative Government in Westminster - made clear it would not accept any arrangement which saw Northern Ireland treated differently from the rest of the UK.
Irish Taioseach Leo Varadkar said he was "surprised and disappointed" by the break-up of talks just hours after all sides confirmed they were satisfied with a text which would guarantee "regulatory alignment" between Northern Ireland and the Republic in order to prevent the imposition of a hard border.
Downing Street made no response to Mr Varadkar's claim.
But it is understood that, as well as the impasse over Ireland, disagreement remains over the role of the European Court of Justice in overseeing EU citizens' rights in the UK after Brexit.
Both Mrs May and Mr Juncker said they were "confident" of reaching agreement in time for a key summit of the European Council on December 14, when it is hoped that leaders of the remaining 27 EU states will give the green light for the start of trade talks.
The Prime Minister is expected back in Brussels for further talks before the end of the week, though sources declined to confirm reports that she would meet Mr Juncker and chief Brexit negotiator Michel Barnier on Wednesday.
European Council president Donald Tusk later disclosed that he had been preparing to move negotiations on to the second phase - dealing with trade and the transition to the post-Brexit EU/UK relationship - until the last-minute call for more time.
Mrs May arrived for her lunchtime talks with Mr Juncker with expectations high for a breakthrough on remaining "divorce" issues which might allow Brexit negotiations to move on to their second phase by Christmas, easing the pressure on businesses to activate contingency plans to move staff and activities outside the UK.
But Irish pronouncements over an imminent deal on the border sparked a firm response from the DUP, which has previously warned it could withdraw its support for Tories in Westminster if a deal is proposed which threatens the integrity of the United Kingdom by effectively establishing a customs border in the Irish Sea.
Speaking at Stormont, DUP leader Arlene Foster said: "We have been very clear. Northern Ireland must leave the EU on the same terms as the rest of the United Kingdom. We will not accept any form of regulatory divergence which separates Northern Ireland economically or politically from the rest of the United Kingdom."
Her comments were swiftly followed by statements from the leaders of devolved administrations in Scotland, Wales and London, making clear that any special status for Northern Ireland would prompt demands from other parts of the UK for their own tailor-made Brexit.
Scottish First Minister Nicola Sturgeon said that if one part of the UK could stay in the single market, there was "no good practical reason why others cannot do the same".
Allowing Northern Ireland alone to retain regulatory alignment with the EU would put Scotland at " a double disadvantage when it comes to jobs and investment", she said.
Wales would "fully expect" to be offered the same deal as Northern Ireland to stay in the single market and customs union, said First Minister Carwyn Jones, while London's Labour Mayor Sadiq Khan said a "similar deal" could help preserve tens of thousands of jobs in the capital, which voted overwhelmingly to Remain in the EU.
Mrs May broke off from talks with Mr Juncker to speak by phone with Mrs Foster, and it was shortly afterwards confirmed that talks were ending without a deal.
The Prime Minister said "a lot of progress" had been made, with "a common understanding" on many of the divorce issues.
But she added: "On a couple of issues some differences do remain which require further negotiation and consultation."
Mr Juncker said that the EU and UK were "narrowing our positions to a huge extent", but that two or three issues remained "open for discussion".
In a press conference in Dublin, Mr Varadkar said that representatives of the UK Government and Mr Barnier's team informed Irish negotiators early in the day that a form of words had been found which might satisfy the Republic's demand for a "cast-iron guarantee" of no hard border after Brexit.
"The Irish negotiating team received confirmation from the British Government and the Barnier taskforce that the United Kingdom had agreed a text on the border that met our concerns," said Mr Varadkar, adding that he confirmed Ireland's support for the text to Mr Juncker and Mr Tusk.
"I am surprised and disappointed that the British Government now appears not to be in a position to conclude what was agreed earlier today," he said.
"I accept that the Prime Minister has asked for more time, and I know that she faces many challenges and I acknowledge that she is negotiating in good faith.
"Ireland wants to proceed to phase two - it's very much in our interests to do so. However we cannot agree to do this unless we have firm guarantees that there will not be a hard border in Ireland under any circumstances."
Labour leader Jeremy Corbyn said the failure of talks showed Mrs May's Government was " completely ill-equipped to negotiate a successful Brexit deal for our country", while former Ukip leader Nigel Farage said she should "leave office now".
But Tory MPs and peers who attended a briefing at Westminster from the PM's chief of staff Gavin Barwell and Brexit minister Steve Baker voiced satisfaction that divisions between Northern Ireland and the mainland had been avoided.
Leading Brexiteer Jacob Rees-Mogg said that moves to "dismantle" the territorial integrity of the UK were "intolerable", while Remain-backing former minister Anna Soubry said that no Tories wanted different rules for different parts of the country and that the simplest way of avoiding this was for the whole UK to stay in the single market.