Confidence early on Monday that an agreement was within reach came to nothing when, during a working lunch with the European commission president, Jean Claude Juncker, May was forced to pause discussions to take a call from Arlene Foster.
Comment: Remember her? That's the leader of the DUP, the only reason Theresa May's Conservatives are still in power in London:
Meet the Democratic Unionist Party (DUP), Britain's loyal fundamentalists in Ireland
The unionist leader, whose party currently provides the Tories with a working majority in the Commons, told the British prime minister that she could not support Downing Street's planned commitment to keep Northern Ireland aligned with EU laws.
In London, Tory Brexiters, including Iain Duncan Smith and Jacob Rees-Mogg, told the Brexit minister Steve Baker, and the prime minister's chief of staff, Gavin Barwell, that they were also rallying behind the DUP's stance.
Lord Trimble, a former first minister of Northern Ireland told the Guardian said Tory MPs at the meeting had shown "unanimous backing" for opposition to the draft proposal he said was "minted in Dublin".
The development raises fresh questions about May's ability to deliver on any deal she proposes to the 27 member states, and has filled diplomats in Brussels with a deep foreboding for future talks, should they develop in the new year to take in the future relationship.
Diplomats were waiting for two hours in a negotiating room at the Council of Ministers headquarters, for a meeting that had been planned to follow the Juncker-May lunch. When it became clear the two sides could not get an agreement, the officials were sent home.
Juncker and May attempted to put a brave face on the spectacular collapse of their plans in press statements at the end of the day. The commission president praised May for being a "tough negotiator" who was energetically fighting for Britain's interests.
"On many of the issues there is a common understanding and crucially it is clear we want to move forward together," May told reporters before hinting further talks were needed. "There are a couple of issues, some differences do remain, which require further negotiation and consultation. And those will continue but we will reconvene before the end of the week and I am also confident we will conclude this positively."
Comment: So for all the media hype about how 'at war' May's cabinet and EU heads are with each other, they're actually being reasonable. The source of the antagonism is the intransigence of Northern Ireland's loyalist leaders.
Labour sources said they understood that a major statement due from the prime minister to parliament on Tuesday had been cancelled. Negotiators are now expected to reassemble by the end of the week, with Wednesday evening now sketched into officials diaries.
The delay also takes the EU into unusual territory, as member states will have less than a week to approve an agreement that many have not yet seen. Some EU diplomats are increasingly concerned about Juncker's extension of the deadline, billed as "the extension of the very last round".
The text of the agreement has to be discussed in 27 national capitals, if EU leaders are to sign it off at a summit on 14-15 December. "The less time we have before the European council, the more difficult it becomes to run the text through 27 national administrations and get an agreement," said one. "It is [the UK's] decision to leave it to the last minute and it is [the UK's] risk."
Government sources made clear that there were two key sticking points yet to be solved in the negotiations with the EU27 - the role of the European court of justice when it came to citizen rights and the Irish border.
On Ireland, one senior government source admitted that progress would be difficult without the support of unionists. "It has to be a deal which is going to carry the support of the EU27 and everybody here. It is a reality that you need DUP support if the deal is to stand the test of time."
In Dublin, where there had been full confidence that an agreement had been reached that could avoid a hard border after Brexit, the Irish prime minister, Leo Varadkar, told reporters that he was "disappointed and surprised" by Britain's U-turn.
Varadkar said: "It is evident that things broke down, became problematic during the lunch in Brussels."
The Irish prime minister's deputy had been on national radio just hours earlier confidently predicting that a deal was close and that "a positive statement for the country" from the taoiseach was planned for the afternoon.
The DUP's fury had been prompted by a leak early on Monday of a draft 15-page joint statement from the European commission and the UK which suggested Britain had bowed to the Republic of Ireland's demands by accepting that "in the absence of agreed solutions the UK will ensure that there continues to be continued regulatory alignment" with the internal market and customs union.
Foster swiftly put out a statement insisting that she would not accept any special status for Northern Ireland as the UK left the EU in March 2019.
The news was then seized upon by Scotland's first minister, Nicola Sturgeon, who suggested that any promise for Northern Ireland could be replicated for Scotland.
That call was followed by similar suggestions from the London mayor, Sadiq Khan.
The European council president, Donald Tusk, tweeted:
The UK wants Brussels to rule that sufficient progress has been made on the opening issues in order for talks on trade and a transition period to open in December, after a summit of leaders next week.