The unscheduled talks in Brussels came amid speculation officials working on the negotiations had reached a deal.
"Despite intense efforts, some key issues are still open, including the backstop for (Ireland/Northern Ireland) to avoid a hard border", Barnier tweeted after his meeting in Brussels with Raab ended days of bargaining between Barnier's deputy Sabine Weyand and May's Brexit lieutenant Oliver Robbins.
But any solution to the border question looked far off. Ireland again said Britain must make good on its commitment to have a backstop plan. "However, there remain a number of unresolved issues relating to the backstop", it read.
BRUSSELS/LONDON, Oct 14 (Reuters) - The stubborn problem of Ireland's land border with Britain defeated marathon talks to seal a Brexit deal on Sunday in time for a European Union summit this week.
But with May under siege from Tory Eurosceptics and her DUP parliamentary allies, her room for manoeuvre is severely restricted.
Former Brexit secretary David Davis has urged a Cabinet revolt against the Prime Minister's approach to the talks.
Officials are trying to hammer out a deal this weekend for a public announcement on Monday and a leaders' summit on Wednesday.
People familiar with both sides of the negotiation, who declined to be named, said there were obstacles to a deal over the thorniest issue of all - how to avoid a border emerging on the island of Ireland without erecting one between Northern Ireland and Britain.
European Union negotiator Michel Barnier said after meeting British Brexit Secretary Dominic Raab that they could still not bridge a gap between his "backstop" demands that Northern Ireland stay in the EU's economic zone if there is a risk that border checks with European Union member Ireland could revive conflict, and London's rejection of any checks on trade between the province and the British mainland.
The prime minister's counter-proposal is for a temporary customs arrangement for the whole United Kingdom, but Tory Brexiteers fear this becoming an open-ended position that would prevent free trade deals with countries around the world.
Despite holding the fate of the Government in her hands, Mrs Foster remains an enigma to most voters outside Northern Ireland who know little of her traumatic childhood.
Health Secretary Matt Hancock sought to play down the potential for Cabinet resignations but was unable to say whether a fixed deadline for any customs arrangement would be written into a deal with Brussels.
May's former deputy Damian Green, told the BBC that the Tory leader was a "very Unionist PM, we are the Conservative and Unionist Party.
A total of 44 letters demanding a vote were reportedly submitted to the Conservative 1922 Committee - just four short of the number required to trigger a ballot.
May in particular is likely to have a tough time selling a deal to the House of Commons, which is divided on the issue of Brexit and on what terms to leave the EU.
Mr Davis is understood to be prepared to run if there is a leadership contest.
Stressing that her party - which has threatened to cause a political crisis by voting down Philip Hammond's Budget if Mrs May gives way to Brussels - was serious, she said: "The DUP's actions this week are not as some have suggested about "flexing muscle".
The trouble with the new proposal is that while it would calm the fears of the DUP - which props up May's government - it risks enraging pro-Brexit members of her party who only reluctantly agreed to any kind of transition in the first place.
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