Prime Minister Theresa May has called for “cool, calm heads” as she insisted a Brexit deal was “still achievable” despite differences with the EU.
She said it was “frustrating” the two sides could not agree how to guarantee no hard border in Northern Ireland.
The issue could not “derail” the chances of striking a deal, she said.
And she sought to reassure critics of her approach that the UK would not end up in “permanent limbo” tied to EU customs rules.
German Chancellor Angela Merkel, meanwhile, warned a deal was looking “more difficult” because of the border question.
“If it doesn’t work out this week, we must continue negotiating, that is clear – but time is pressing,” she added.
In the Commons, Mrs May faced pressure from both sides of the Brexit debate as she gave a statement to MPs.
Brexiteers warned of the UK being tied to EU rules, Remainers called for another referendum and the DUP demanded no separate arrangements for Northern Ireland.
Jeremy Corbyn said she should make way for Labour rather than be “buffeted this way and that way by the chaos of her own party”.
Unscheduled talks with the EU on Sunday broke up over the Irish border issue with just days left before the summit which begins on Wednesday.
Mrs May has dismissed the EU’s previous suggestion of keeping Northern Ireland aligned to its trade rules as a safety net, or “backstop” to avoid a hard border, saying this would divide the UK.
She told MPs the EU had “responded positively” to her proposal for a UK-wide arrangement – but that it was insisting on keeping a specific arrangement for Northern Ireland on the table.
Despite this, she said she did not think the two sides were “far apart”, adding: “I continue to believe a negotiated deal is the best outcome for the UK and the EU and that such a deal is achievable, and that is the spirit in which I will continue to work with our European partners.”
Some Brexiteers are unhappy at the prospect of the UK staying inside the EU’s customs regime beyond the proposed transition period, which will end in December 2020.
“I need to be able to look the British people in the eye and say this backstop is a temporary solution,” Mrs May told MPs.
“People are rightly concerned that what is only meant to be temporary could become a permanent limbo – with no new relationship between the UK and the EU ever agreed.”
She said she hoped a backstop would not be needed, as the two sides could agree a trade deal that solved the border issue, potentially as early as January 2021.
But if it is needed, she said it had to be temporary and could not continue indefinitely if the EU does not co-operate on trade talks.
What is the Irish ‘backstop’?
The UK is leaving the EU in March 2019, along with its single market and customs union, which allow for friction-free trade between members.
After Brexit, it will have a land border with the EU between Northern Ireland and the Republic.
Both the UK and the EU want to avoid a “hard border” – physical checks or infrastructure between Northern Ireland and Ireland – but cannot agree how.
So, the backstop is a position of last resort – to protect an open border on the island of Ireland in the event that the UK leaves the EU without having agreed a solution as part of trade negotiations.
The two sides do not agree on what this safety net should look like, however. The EU has suggested Northern Ireland stays aligned with its trade rules so new border checks are not needed.
But Mrs May has said this would undermine the integrity of the UK by creating a new border in the Irish Sea.
She has suggested the UK as a whole could remain aligned with the EU customs union for a limited time after 2020, when the planned transition period ends.
But the EU says a backstop would not work if it is time-limited.
Some Tory Brexiteers say the backstop is not necessary at all because technological solutions can avoid a hard border.