By Costas Pitas and Andrew MacAskill
British Prime Minister Theresa May struggled on Friday to find consensus on a Brexit plan that would be acceptable to her ministers, her divided Conservative Party and the Northern Irish lawmakers who prop up her minority government.
Brexit negotiations with the European Union have accelerated and become more positive over the past week, though significant hurdles remain, finance minister Philip Hammond said.
"What has happened over the last week, 10 days, is that there has been a measurable change in pace," he told the BBC.
"But that shouldn't conceal the fact that we still have some big differences left to resolve," Hammond said. "So process is a lot more positive this week - substance still very challenging."
With less than six months to go until the United Kingdom is due to leave the EU, its most momentous shift in foreign and trade policy for over four decades, May is seeking to rally support at home on the details of a divorce deal though it is unclear if she can win parliamentary approval.
The Irish border "backstop", which seeks to avoid extensive checks on the frontier between the British province of Northern Ireland and EU state Ireland should the open border there not be maintained by a new, post-Brexit EU-UK trade partnership, is the biggest sticking point.
The EU side has indicated progress on future management of the 500-km-long border but diplomats in Brussels signaled the lifespan of any such emergency frontier fix was still not agreed. Negotiators were also sparring over whether that would cover Northern Ireland or mainland Britain as well.
"It would have to be finite, it would have to be short and it would have to be, I think, time-limited in order for it to be supported here," said Britain's Brexit minister, Dominic Raab.
"What we cannot do is see the United Kingdom locked in via the backdoor to a customs union arrangement which would leave us in an indefinite limbo - that would not be leaving the EU," he said on Friday.
The EU says a backstop must be "all-weather" and not have a specific cut-off date. Negotiators are looking for creative wording to square that circle.
They will go on through the weekend and diplomatic sources in Brussels expect Raab to come on Monday if there is a deal. EU advisers of the 27 member states staying on in the EU after Brexit are also due to meet in Brussels that day.
The bloc's national leaders meet for a high-stakes summit on Wednesday, hoping to declare "decisive progress" in the divorce talks and announce another extra Brexit summit to finalize the deal, including an offer of close future ties with Britain.
But for all the cautious positivity coming from the EU side, May's Northern Irish supporters vehemently oppose any post-Brexit checks between the province and the rest of Britain.
The head of the Democratic Unionist Party, Arlene Foster, said May "could not in good conscience" agree to checks on goods being imported to Northern Ireland from Britain after Brexit.
Under May's plan, the whole of the United Kingdom would forge a customs "partnership" with the EU after a transition period ends in December 2020 if the backstop is triggered. Some of May's ministers have urged a time limit on that.
"The prime minister would never agree to a deal which could trap the UK in a backstop permanently," May's spokeswoman said on Friday.
The Times newspaper reported that May could face further resignations by euroskeptic ministers in her cabinet unless she found a way to ensure the backstop was not permanent.
In the worst-case scenario of Britain failing to agree any divorce deal, London warned that the single electricity market shared between Northern Ireland and the Irish Republic may cease. Consumers may also no longer enjoy the protections they are used to when buying goods and services in the EU, it added.
Britain warned on Friday that leaving the EU without a deal would mean drugmakers needing to stockpile experimental treatments in case of border delays. Professional qualifications from European nations may no longer be honored.
London has published more than 25 technical notices covering issues from consumer rights and sanctions policy to rail transport and taking horses abroad. It said it was working hard to mitigate risks in these areas in case of a no-deal.
(Additional reporting by Kate Holton, Padraic Halpin and David Milliken in London, Gabriela Baczynska, Philip Blenkinsop and Alastair Macdonald in Brussels, Writing by Guy Faulconbridge and Gabriela Baczynska, Editing by Mark Heinrich)