MPs are to discuss how the government’s Brexit deal affects the union when the penultimate day of debate begins later.
Cabinet Office Minister David Lidington will open the debate, with NI Secretary Karen Bradley due to close it.
Some MPs, including the Democratic Unionist Party (DUP), say the deal threatens the integrity of the union because of the Irish border backstop.
But the prime minister has insisted her plan protects commitments made to NI in the Good Friday Agreement.
Why is the union part of the Brexit debate?
Both the EU and UK have committed to ensuring the border between Northern Ireland and the Republic of Ireland remains as frictionless after Brexit as it is now.
The government’s deal contains the Northern Ireland protocol: the insurance policy to avoid the return of a hard border if a solution cannot be reached through a wider trade deal.
Also known as the backstop, it sparked a political backlash amid claims it could lead to the break up of the United Kingdom.
That is because if the backstop took effect, Northern Ireland alone would align with the EU single market in some areas, meaning new regulatory barriers between GB and NI, effectively creating an Irish Sea border.
The DUP and Brexiteer MPs have vowed to reject the deal unless the backstop is ditched, because they say any differences for Northern Ireland could threaten the union and damage the economy.
The UK would also not be able to leave the backstop without EU agreement.
However, Sinn Féin, the SDLP, the Alliance Party and the Greens, as well as a number of business and agri-food groups, have urged Parliament to back the deal.
What is the DUP saying?
The DUP is “focused on defeating” the government’s Brexit deal, party leader Arlene Foster has said.
Mrs Foster told BBC Radio Ulster on Monday that the DUP was “talking to Labour about the fact this is a bad deal”.
She also rejected suggestions the DUP could back a Norway Plus-style deal – which would remove the need for a backstop but would see the UK remaining in the European Economic Area (EEA) and joining a customs union with the EU.
However, the UK would have to accept free movement of people – breaching a previous “red line” laid down by the PM.
Mrs Foster said Norway Plus “puts us in the customs union and doesn’t give us control of our laws”.
She also would not be drawn on whether the prime minister should be replaced in order to negotiate a new deal, adding: “Our focus is not on who leads the Tory party but defeating this deal”.
The DUP leader insisted there is “still time” to work for a better deal and said that was only possible if the government ditched the backstop proposal.
What’s the latest on the backstop?
Boris Johnson, who quit the cabinet over Mrs May’s Brexit strategy, told the BBC the Northern Ireland “backstop” put the UK in a “diabolical negotiating position”.
MPs could give Mrs May “a powerful mandate to change that backstop” by voting it down on Tuesday, he said.
Cabinet ministers have been trying to rally support for it, with cabinet office minister David Lidington visiting Belfast last Friday, but a proposed compromise was dismissed by the DUP and Brexiteers.
A Tory backbench amendment, which has government support, suggested MPs could be “given a role” in deciding whether to activate the backstop.
It would also give the devolved administrations, particularly the Northern Ireland Assembly, more say in the process – but there has been no power-sharing executive at Stormont since 2017, when the institutions collapsed.
With the government widely expected to lose Tuesday’s Commons vote, Downing Street has rejected calls for it to be delayed.
Theresa May has insisted there will be no deal on offer from the EU that does not include a backstop.
But the DUP – which props up the government in a confidence-and-supply pact to give Mrs May a working majority – is putting pressure on the government to “bin the backstop” or risk a huge defeat.
Its MPs have already flexed their political muscle by voting against the government several times, in a bid to force the prime minister to change course.
Taoiseach (Irish prime minister) Leo Varadkar spoke to Mrs May by phone on Sunday night. A spokesman for the Irish government said among the things discussed was Tuesday’s Commons vote on the deal.
What about Scotland and the union?
Since the withdrawal deal was published, Scottish politicians have criticised it on two fronts.
Scottish Conservative MPs fear any new controls that separate Northern Ireland from the rest of the UK could further fuel the case for Scottish independence.
Scotland’s First Minister Nicola Sturgeon has said her party’s MPs will also vote against the deal in Parliament.
She said the backstop could leave Scotland at a “serious competitive disadvantage” to Northern Ireland, if it is able to access both UK and EU markets more easily than Scotland.
The SNP leader also said the case for Scottish independence “has never been stronger”, and said she would return to this issue “when we know which approach to Brexit the House of Commons chooses”.
What happens next?
MPs can propose amendments to the deal until Monday evening – but it is up to the Speaker to determine which ones are selected for debate.
Day four of debate on the deal is due to start on Monday afternoon and run late into the evening.
There will be a final day for MPs to discuss the deal on Tuesday, before the vote is set to happen that night.
There are a number of possible outcomes if MPs fail to back the agreement – six of the most likely ones are here.