The EU's suggestion would see Northern Ireland remain aligned with Brussels' rules, thus varying from the rest of the UK
British Prime Minister Theresa May faces a fight on two fronts this week, battling to convince her own ministers and then Brussels as the Brexit talks come to a crunch.
She must see off the threat of a cabinet mutiny and then try to overcome the divorce negotiations logjam at a summit in Brussels -- though a breakthrough still seems elusive.
Time is running out on Britain's EU exit talks -- meaning this week's gathering of European Union leaders could prove decisive in striking a deal between London and Brussels.
With Britain set to leave the bloc at the end of March, European Commission head Jean-Claude Juncker is demanding "substantial progress" this week, specifically on the vexed issue of the UK's border with the Republic of Ireland.
EU President Donald Tusk has described the summit starting Wednesday as a "moment of truth" in the Brexit talks.
As for May, she not only has to win over her continental counterparts but also increasingly restive allies back home.
The hard work starts for May on Tuesday when she will rake over the Irish border issue with her cabinet, amid speculation that further ministers could quit if the PM ploughs on with her proposals.
David Davis, who quit as Brexit secretary in July over May's broad blueprint, wrote in The Sunday Times newspaper that her plans were "completely unacceptable" and urged ministers to "exert their collective authority" this week.
Neither London, Dublin nor Brussels wants to see checks imposed on the border between the UK's Northern Ireland and the Irish Republic -- but the problem persists of finding a way to square that aim with May's desire to leave the European single market and the customs union.
Britain has proposed that it would continue to follow EU customs rules after Brexit as a fall-back option to keep the border open, until a wider trade deal is agreed that avoids the need for frontier checks.
May says this will only be temporary, but her spokeswoman was forced to clarify the point after media reports that the final "backstop" arrangement will have no legal end date.
The EU's suggestion would see Northern Ireland remain aligned with Brussels' rules, thus varying from the rest of the UK.
The plans have infuriated the pro-Brexit hardcore of May's centre-right Conservative Party -- not to mention her Northern Irish allies in the Democratic Unionist Party.
For a thin majority in parliament, the minority Conservative government relies on the DUP, Northern Ireland's biggest party.
The DUP are staunchly pro-UK, pro-Brexit and opposed to any moves that could put distance between Northern Ireland and the British mainland.
The DUP have threatened to vote down the British government's budget if May gives way to Brussels.
Writing Saturday in the Belfast Telegraph newspaper, DUP leader Arlene Foster said they were not simply "flexing muscle" for the sake of it.
"This backstop arrangement would not be temporary. It would be the permanent annexation of Northern Ireland away from the rest of the United Kingdom," she wrote.
May must not accept "a dodgy deal foisted on her by other,” she added.
After talks with senior figures in Brussels last week, Foster reportedly said a no-deal Brexit was now the most likely outcome, according to an email between senior British officials leaked to The Observer newspaper.
Meanwhile leading Conservative eurosceptic Jacob Rees-Mogg said Saturday there were 39 like-minded Conservative MPs who "will not turn" from opposing the current plans.
"The prize is there to be grasped," he said, adding that Britain could not accept a "punishment Brexit."
"If the EU is a mafia-style organisation that says 'if you want to leave, we will kneecap you', then all the better for leaving," he said.
Some diplomats in Brussels have suggested the leaders could talk through the night on Wednesday and approve the outlines of an agreement while they are still in the Belgian capital for broader talks on Thursday.