Theresa May faces a crunch cabinet meeting later when she will try to win colleagues’ support for a draft Brexit agreement between the UK and Brussels.
Senior ministers will gather at Downing Street at 14:00 GMT amid calls to reject the deal from senior Brexiteers and Remain supporters.
Some cabinet members have “deep reservations”, BBC political editor Laura Kuenssberg said.
The PM is continuing one-to-one briefings with ministers on the plans.
Meanwhile, ambassadors from the remaining 27 EU member states will discuss the agreement and the possibility of organising an emergency summit to agree it.
If this happens, the government will then face a battle to win Parliament’s backing, with some Tories vowing to vote against it and Northern Ireland’s Democratic Unionist Party (DUP) also expressing concern.
Leading Brexiteer Jacob Rees-Mogg told BBC Newsnight he was so unhappy with the agreement he could withdraw his backing for Mrs May.
But Conservative Chief Whip Julian Smith said he was “confident” the deal would pass when put to a crucial Commons vote.
What’s been agreed?
The draft withdrawal agreement, which has been drawn up alongside a statement of future relations with the EU, has not been published but is thought to run to some 500 pages.
It addresses the Northern Ireland “backstop”, which aims to guarantee that physical checks will not be reintroduced at the border with the Irish Republic, in the event of the EU and UK failing to agree a deal on future trading relations.
This has proven the most contentious part of the withdrawal negotiations, with concerns raised by Brexiteer Tories and the DUP over how it will work.
The backstop within the agreed draft is believed to avoid a return to a “hard border” with the Republic by keeping the UK as a whole aligned with the EU customs union for a limited time.
However, some Brexiteers fear this will keep the UK locked into EU trade rules for years.
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The agreement also includes commitments over citizens’ rights after Brexit, a proposed 21-month transition period after the UK’s departure on 29 March 2019 and details of the so-called £39bn “divorce bill”.
The future relationship statement is expected to be far shorter, with the UK and the EU’s long-term trade arrangements yet to be settled.
Number 10 said ministers were being called to the emergency meeting to “consider the draft agreement the negotiating teams have reached in Brussels, and to decide on next steps”.
Before they do, they will be able to read relevant “documentation”.
What are people saying?
Former foreign secretary Boris Johnson described the reported agreement over the Northern Ireland backstop as “utterly unacceptable to anyone who believes in democracy”, and said he would vote against it.
Mr Rees-Mogg told BBC Newsnight it was “not what we were promised”, saying he could find it “very hard to carry on supporting” the prime minister.
And former Brexit Secretary David Davis said: “Cabinet and all Conservative MPs should stand up, be counted and say no to this capitulation.”
But one of Mrs May’s allies, former first secretary of state Damian Green, criticised “hyperbole” from people who had not read the document.
By BBC political editor Laura Kuenssberg
It doesn’t seem to me that many of the cabinet are likely to walk on Wednesday over what’s in the document.
It’s suggested that those with bigger doubts are more likely to cause problems for the prime minister because it won’t get through Parliament.
One source told me senior ministers are thinking not just about the wisdom of backing a deal they don’t like because it’s a sour compromise, but whether it is folly to back a deal they believe can’t get through Parliament.
Slamming on the brakes now would force a crisis, but it could be less serious than the political disaster of pursuing this plan to an eventual calamitous defeat that could take them all down.
The DUP, whose support Mrs May needs to win key votes, said its desire for a deal “will not be superseded by a willingness to accept any deal”.
Party leader Arlene Foster warned it would be “democratically unacceptable” for Northern Ireland’s trade rules to be set out by Brussels, and that “without a clean exit, the UK will be handcuffed to the EU”.
Labour leader Jeremy Corbyn said “given the shambolic nature of the negotiations, this is unlikely to be a good deal for the country”.
Meanwhile, pro-EU Conservative MP Justine Greening said the agreement would leave the UK with less influence and undermine its credibility.
Speaking at a rally calling for another EU referendum to be held, she said: “Even if some people in my party can’t see this is a bad deal, everyone else around this entire planet can.”
Former Transport Minister Jo Johnson told the audience cabinet ministers were “looking deep into their consciences” about whether to support the deal.
View from the EU
By BBC Europe editor Katya Adler
What’s so striking about this draft Brexit deal the UK media and politicians are all abuzz about, is the marked lack of excitement and/or hysteria in EU circles.
Contrary to the UK narrative, this is not viewed in Brussels as the back-against-the-wall, make-or-break moment.
There’s still some time to keep negotiating. EU-UK technical talks are, in fact, ongoing as neither all the “i”s, nor all the “t”s of a deal have yet been dotted or crossed.
The thinking here is: if the UK cabinet or certain EU member states strongly object to specific parts of the draft document (as long as they don’t rip up the whole thing), then negotiators can go back to the drawing board.
UK PM faces cabinet showdown over Brexit deal