The European Union has published a draft of the Brexit withdrawal agreement to ensure Britain's "orderly" departure from the bloc in March 2019.
Here, we answer key questions about the document:
- What is in the document?
The draft exit deal includes provisions around so-called "divorce" issues such as citizens' rights, a financial settlement and the Irish border, alongside plans for a largely "status quo" post-Brexit transition period to last to the end of December 2020.
EU chief negotiator Michel Barnier has insisted there are "no surprises" in the text because it sets out Brussels' previously stated aims on transition and translates political commitments agreed by both sides in December.
- But are some parts proving controversial?
A few - mainly the plan to keep Northern Ireland in a "common regulatory area" with the EU to maintain a soft border with the Republic of Ireland if no other solution can be found, in what has been described as a de facto customs union.
- What's wrong with that?
Mr Barnier has said the proposal simply puts into legal terms the December Joint Report agreement on "regulatory alignment" struck by Theresa May and European Commission president Jean-Claude Juncker.
The negotiator said that it is simply a backstop power to maintain a soft Irish border if the issue cannot be resolved by a free trade agreement or if the UK fails to find technological or diplomatic solutions.
But the detailed and stark legal terms have been rejected by Mrs May, who said "no UK prime minster could ever agree to it" as it "threatens the constitutional integrity" of Britain by suggesting a new de facto border would be drawn in the Irish Sea between Northern Ireland and England, Wales and Scotland.
- How about the other issues?
Brussels' plan to make the European Court of Justice the ultimate arbiter in resolving disputes relating to breaches of the agreement, if a joint EU-UK committee cannot sort them out, breaches a red line set by the Prime Minister at the beginning of the Brexit process.
The insistence that EU citizens who arrive in Britain during the transition have the same right to stay in the country as those who arrived before Brexit is also likely to run into opposition from Mrs May, who said during a January trip to China that free movement rights must end after Brexit.
And a proposal to give the EU the power to sanction the UK during the transition by suspending certain benefits of the single market, if Brussels believes the terms of agreement has been broken, has already rankled in Westminster.
- What happens next?
Mr Barnier stressed that the document was a draft and would now be circulated among the 27 other EU nations and the European Parliament's Brexit steering group for revision and agreement before being placed on the negotiating table.
Mrs May has made clear she will not agree to it in its current form, but attention will now turn to her major speech on Friday in which she will set out the UK's strategy for the next phase of negotiations on trade.
The document adds to the pressure on the PM to perform a U-turn and commit to a customs union with the EU to solve the Irish border problem and ensure tariff-free trade in many areas after Brexit.