British Prime Minister Theresa May is expected to reveal Monday what changes, if any, she has secured to her EU divorce deal, on the eve of a crucial vote in parliament less than three weeks before Brexit day.
Two months ago, parliament overwhelmingly rejected the withdrawal agreement May has struck with the European Union, and sent her back to Brussels to renegotiate.
But as MPs prepare to vote again on the deal on Tuesday, the prime minister has little to show for her efforts, prompting warnings of another humiliating defeat.
Failure means Britain could end 46 years of ties with its closest trading partner on March 29 with no new arrangements in place, causing huge disruption on both sides of the Channel.
But it would also raise the possibility of a delay to Brexit.
While europhiles in May's Conservative Party would welcome a delay as a possible precursor to a second referendum on EU membership, eurosceptics strongly oppose it.
'Eight weeks of failure'
British and European officials worked through the weekend to try to break the deadlock, and May is ready to make a last-minute visit to Brussels if necessary to seal the deal.
But the EU has already rejected most of her demands, suggesting that any concessions she does get will not be enough to convince British lawmakers.
The opposition Labour's Brexit spokesman Keir Starmer on Sunday railed against May for "eight weeks of failure", saying she was coming back with "exactly what was rejected".
Ministers concede the deal is not perfect but say it is the best way to move forward, and warn rejecting it could put Brexit at risk.
In the face of a cabinet revolt, May has promised that if MPs defeat her plan, they will be able to vote this week on whether to leave the EU with no deal, or seek a delay.
"There is a risk and possibility that we end up losing Brexit if we get the votes wrong in the next couple of weeks," Foreign Secretary Jeremy Hunt told the BBC on Sunday.
The deal was struck over more than a year of tough negotiations, and covers Britain's financial settlement, expatriate rights, the Irish border and plans for a transition period.
But MPs on all sides in London were swift to condemn it for a variety of reasons, and it was rejected in January by 432 votes to 202.
MPs then voted to ask May to seek changes to the most controversial element, the backstop arrangement intended to keep the Irish border open after Brexit.
This would keep Britain in the EU's customs union and parts of its single market until and unless another way - such as a trade deal - is found to avoid frontier checks.
Many MPs fear it is a trap to keep them tied to EU rules, but Brussels has rejected calls for a time limit or unilateral exit clause.
EU chief negotiator Michel Barnier repeated on Friday that the bloc could offer a legally binding statement confirming the backstop was only meant to be temporary.
But this offer from the Frenchman - who spent Sunday watching the rugby Six Nations in Dublin with Irish premier Leo Varadkar - is unlikely to be enough.
"It is inevitable this unchanged withdrawal agreement will be voted down again," two senior Brexit-supporting MPs, Steve Baker and Nigel Dodds, warned on Sunday.
Fellow MP Jacob Rees-Mogg, an influential Brexit supporter, wrote in the Express: "Leaving without an agreement is nothing to be frightened about. It opens the door to prosperity".
If the deal is rejected, May has promised a vote on Thursday on a short delay to Brexit, but warned MPs they could not put off taking a decision.
Any delay would have to be approved by the leaders of the other 27 nations, who are next meeting at a Brussels summit on March 21-22 - a week before Brexit day.
Former Brexit minister David Davis said that postponing Britain's departure would let down all those who voted to leave the EU in the 2016 referendum.
It would be a "democratic disaster", he told the BBC on Sunday, adding: "Britain would get its Trump moment."