Prime Minister Theresa May on Tuesday faced her divided ministers as negotiators scrambled to secure a divorce agreement with the European Union and anxiety mounted over the risk of a no-deal Brexit.
May said Monday her government was working "extremely hard" to save a deal, telling officials of London's financial district that negotiations "are now in the endgame".
A British government source said a deal must happen by Wednesday if there is any hope of an extraordinary EU summit this month to sign a withdrawal agreement.
May's de facto deputy David Lidington told BBC Radio's Today programme on Tuesday that a deal was "not quite there yet" but was within "touching distance".
Agreement was still to be reached on a "small number of difficult issues", he added, saying a deal before Wednesday was "still possible, but not at all definite".
May addressed City of London leaders on Monday, saying her negotiators were working "through the night" to make progress on the remaining issues.
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"I will not compromise on what people voted for in the referendum. This will not be an agreement at any cost," she added.
But pro-Brexit ministers in the cabinet are worried that May is set to agree to the EU's demands over the key sticking point of the Irish border.
At issue is how to avoid border checks between British Northern Ireland and EU member the Republic of Ireland after Brexit enters into force from March 29.
London suggests Britain could temporarily stay aligned with the bloc's trade rules but wants to reserve the right to exit the arrangement.
The EU appears only ready to accept that if there is a fallback option written into the withdrawal agreement in which Northern Ireland would remain in the European single market while the rest of Britain leaves.
Ministers are set to confront May during Tuesday's meeting, warning that the EU's demands were "completely unacceptable", according to the Daily Telegraph.
Former Brexit minister David Davis, now a strident critic of May's strategy, warned that the government "should be prepared to exit without a deal".
Failure to strike a deal by Wednesday could push back the signing of any agreement until a regular EU summit on December 13, dramatically curtailing the time May will have to get the deal past a rebellious British parliament before Brexit day on March 29.
In parliament, May will have to satisfy both pro-Brexit and anti-Brexit wings of her party, as well as Northern Ireland's DUP party that props up her government.
As a sign of the battles ahead, eurosceptics within her party are demanding in parliament that she lays out "the government's political and legal position" on the various Brexit scenarios.
Labour is expected to go a step further on Tuesday and demand the publication of the full legal advice given to the government by attorney general Geoffrey Cox.
The deal is meant to finalise Britain's exit bill of around 39 billion pounds (USD 50 billion).
It must also guarantee citizens' rights and launch a 21-month transition during which London would follow EU rules.
British and European negotiators will launch more ambitious talks during this transition period on a future trading and legal relationship.
Britain will be forced to ramp up its plans for a no-deal Brexit if a deal is not struck this week.
Amid the uncertainty, EU nationals appear to be voting with their feet. Official figures released Tuesday revealed that the number of EU nationals working in Britain fell by 132,000 to 2.25 million, the largest annual fall since comparable records began in 1997.