Prime Minister Theresa Mays office insisted Friday it would not "trap" Britain in an endless customs union with the EU after Brexit, amid reports some ministers could quit if this is the price of a divorce deal.
But her spokeswoman declined to confirm that a fall-back plan to keep open the land border with Ireland after Britain leaves the bloc in March 2019 would be "time-limited".
Several eurosceptic ministers are now reportedly considering resigning over plans to give ground on the so-called backstop arrangement.
Britain has proposed a customs arrangement with the EU to avoid physical checks between Northern Ireland and EU member Ireland, until the issue can be resolved with a wider trade deal.
A Downing Street spokeswoman emphasised Friday that this would be "temporary", saying: "The prime minster would never agree to a deal which could trap the UK in a backstop permanently."
She repeated that Britain wanted a new trade deal by the end of December 2021 at the latest.
However, she declined to say the backstop would be "time-limited", a phrase which was in the original proposal published by London in June.
Brussels has insisted the backstop cannot by its very nature be time-limited.
However, British eurosceptics are wary of being tied to the bloc indefinitely.
Threat to bring down government
May is also facing a revolt by her Northern Irish allies over the long-term trading relationship, an outline of which London wants set out before Brexit.
The Democratic Unionist Party (DUP) has threatened to bring down the government if the deal results in new barriers to trade between Northern Ireland and mainland Britain.
DUP MP Nigel Dodds said on Friday his party also needed any backstop to be time limited.
"This is critical. If it isn't then the UK including Northen Ireland would be trapped in unacceptable arrangements unless and until the EU decide otherwise," he tweeted.
A full meeting of the cabinet is due next Tuesday, the day before May heads to Brussels to discuss Brexit with the other 27 EU leaders.
Both sides had originally set the October 18 summit as the deadline for a divorce deal, but the timetable is slipping, with another summit in November now looking likely.
"There are still big issues remaining and we will continue to talk today and over the weekend if necessary," the Downing Street spokeswoman said.
Finance minister Philip Hammond said there had been a "measurable change in pace" in talks in the last 10 days, but there were "some big differences left to resolve".
If a deal is done, Britain could enjoy a "deal dividend" which would "deliver us an upside" for the economy, he told the BBC from a meeting of the World Bank and IMF in Bali.