Denis Staunton, the London editor of the Irish Times, accuses Theresa May of "dithering" on the border issue, and warns that she could come under pressure from the DUP and Brexit supporters in her own party to "walk away from talks".
The unraveling of the U.K.'s divorce deal came about when the Democratic Unionist Party - which props up May's government - refused to sign off on a plan to keep Northern Ireland in "regulatory alignment" with the European Union in an effort to keep an open, "frictionless" border with Ireland.
"We're not quite yet where we need to be". The PM will meet European Commission president Jean-Claude Juncker in Brussels, with Downing Street saying "plenty of discussions" lie ahead.
If the trade offer cannot be made at the December 15 summit, a senior European Union official said, the whole process might have to be put back until February - a delay which could increase pressure at home on May and raise a risk of disrupting the existing process.
"In the final stretch of these complex negotiations, the prime minister is on a razor edge", he writes, and wonders whether Mrs May's latest concession to Brussels might now "unpin the Brexit hand grenade in the UK" and "ignite Belfast, and then Edinburgh and Cardiff" in revolt against her planned deal.
Since the referendum in 2016, high-profile opponents of Brexit have suggested Britain could change its mind and avoid what they say will be a disaster for its economy.
Bound by previous EU internal agreements not to share the draft until May has formally committed Britain to covering outstanding EU payments, guaranteeing EU citizens' rights and an open border for Northern Ireland, Tusk must now wait until May returns and EU negotiator Michel Barnier has signed off a deal. But any solution will need the support of Northern Ireland's pro-Brexit Democratic Unionist Party, whose 10 members of parliament are propping up May's government.
Ireland has called on Britain to provide details of how it will ensure there is no "regulatory divergence" after Brexit in March 2019 that would require physical border controls. "The Irish government remains hopeful", the official said. Irish Deputy Prime Minister Simon Coveney said on Sunday his country had "no desire" to delay the UK's Brexit talks, although not enough progress had been made so far.