The UK and EU have failed to reach an agreement to move to the next stage of Brexit talks, Theresa May has said.
The prime minister said differences remain on a “couple of issues”.
She said talks would reconvene “before the end of the week” and she was “confident we will conclude this positively”.
The BBC’s Laura Kuenssberg said the deal had been “sunk” by the DUP, which reacted angrily to reports of concessions on the Irish border issue.
Mrs May is understood to have broken off from talks with European Commission President Jean Claude Junker to speak to Arlene Foster, after the DUP leader had held a press conference saying her party “will not accept any form of regulatory divergence” that separates Northern Ireland from the rest of the UK.
“During the call it was made plain to the PM that the DUP, whose support is vital to the government being able to pass their Brexit legislation, had significant concerns about the deal being discussed that gave concessions to the Dublin government,” the BBC’s political editor said.
“I understand Foster told Theresa May that she would not be able to support such a deal. It’s been suggested too that there are 20 or so Conservative MPs who had serious misgivings about the compromises that were understood to be on the table.”
The UK was reportedly prepared to accept that Northern Ireland may remain in the EU’s customs union and single market in all but name.
The EU has said it will not talk about Britain’s post-Brexit trading relations until an agreement has been reached on the issue of citizens’ rights, how much the UK will pay as it leaves – the so-called “divorce bill” – and the Irish land border.
The prime minister is attempting to reach an agreement on these three issues ahead of a summit of EU leaders in 10 days’ time.
Speaking at a joint press conference in Brussels with Mr Juncker, Mrs May said: “We have been negotiating hard. And a lot of progress has been made. And on many of the issues there is a common understanding.
“And it is clear, crucially, that we want to move forward together. But on a couple of issues some differences do remain which require further negotiation and consultation.”
Mr Juncker said “it was not possible to reach complete agreement today” despite their “best efforts”.
But he added: “I have to say that we were narrowing our positions to a huge extent today, thanks to the British prime minister, thanks to the willingness of the European Commission to have a fair deal with Britain.
“I’m still confident that we can reach sufficient progress before the European Council of 15 December.
“This is not a failure, this is the start of the very last round. I’m very confident that we will reach an agreement in the course of this week.”
Scottish First Minister Nicola Sturgeon said that if Northern Ireland was allowed to operate under different rules there was “surely no good practical reason” why other parts of the UK could not do the same – a message echoed by Welsh First Minister Carwyn Jones and London Mayor Sadiq Khan.