The Irish border question took center stage in Britain's negotiations to leave the European Union on Monday, causing a major setback for Prime Minister Theresa May, who had hoped to conclude an initial agreement but instead was forced to acknowledge talks had faltered.
Eighteen months after the country voted to leave the EU — a referendum result now colloquially referred to as Brexit — both May and her European counterparts had initially appeared upbeat about the prospect of concluding the first round of talks as they arrived in Brussels.
But after a day of talks in Brussels, she stood on a podium beside European Commission President Jean-Claude Juncker and declared that attempts at an agreement had failed, for now.
"On a couple of issues some differences do remain which require further negotiation and consultation," she acknowledged, trying to downplay the intense politicking that had unfolded throughout the day.
At stake were three key issues: How much Britain should pay the EU to sever ties, a so-called divorce bill; the status of EU citizens living in Europe as well as British citizens living in the EU; and what the border will look like between Northern Ireland, which must leave the EU along with the rest of the United Kingdom, and mainland Ireland, which is part of the European Union.
This final issue proved to be the trickiest on Monday.
Reports suggest that May and Juncker were willing to accept a deal that would in essence have seen Northern Ireland remain within the EU's single market and customs union, even though the rest of the United Kingdom is likely to lose such privileges.
Such a deal would prevent a hard border from reemerging between the two parts of Ireland and ensure that smooth trade relations could be maintained.
But statements from the Democratic Unionist Party, or DUP, the group that May's government now depends on to maintain a majority in Parliament, were quick to thwart that.
"We will not accept any form of regulatory divergence which separates Northern Ireland economically or politically from the rest of the U.K.," DUP leader Arlene Foster said in a news conference Monday afternoon.
"The Republic of Ireland for their part claim to be guarantors of the Belfast Agreement, but they are clearly seeking to unilaterally change that Belfast Agreement without our input or consent, and of course we will not stand for that."
The Belfast, or Good Friday, Agreement is the deal that finally brought peace to Northern Ireland after 30 years of sectarian conflict.
A short while later, May reportedly broke off her talks with Juncker to speak by phone with Foster, and later returned to the room to say the deal was off.
Even as the DUP threw a wrench into the talks, the governments in Scotland and Wales, as well as London Mayor Sadiq Khan, were also quick to state that if Northern Ireland got preferential treatment, they expected a similar deal.
"Huge ramifications for London if Theresa May has conceded that it's possible for part of the UK to remain within the single market & customs union after Brexit," Khan wrote on Twitter. "Londoners overwhelmingly voted to remain in the EU and a similar deal here could protect tens of thousands of jobs."
Irish Prime Minister Leo Varadkar said he was "surprised and disappointed" by the day's developments, which came after weeks of negotiations between the British government and the EU.
Varadkar said his government is seeking at all costs to prevent a hard border with Northern Ireland, which some analysts fear could reignite the sectarian tensions that engulfed the region for so many decades.
But the difficulties in cutting a deal on this issue are a sign of the scale of the quagmire Brexit is proving to be for May, who came to power after the referendum in which 48% of Britons voted to remain part of the EU while 52% voted to leave. Her predecessor David Cameron resigned as prime minister after staking his political career on backing the remain side, and May promised to unite the deeply divided country.
But in some ways those divisions have only intensified.
The markets responded Monday with the pound falling as soon as it became clear no deal was being reached.
Speaking alongside May at the afternoon's news conference, Juncker struck a conciliatory tone and even praised May's skills as a negotiator.
He said there had been "significant progress," and a deal on the first round of negotiations was getting closer, a crucial milestone as it will enable Britain to move on to talking about trade.
"This is not a failure, this is the start of the very last round," he said. "I'm very confident we will reach an agreement in the course of this week."
Instead of leaving Brussels with a deal under her belt on Monday, May is now expected to have to return later this week to try to seal it.
It is hoped that this will pave the way for a summit with the remaining 27 EU countries on Dec. 14, where it is hoped they will be able to give the go-ahead for future talks.
Boyle is a special correspondent.
2:15 p.m.: This story was updated throughout with staff reporting.