MPs are preparing to vote on amendments to Theresa May’s Brexit deal that could shape the next stage of negotiations with the EU.
Conservative MPs have been told to back a proposal for an alternative to the Irish backstop – the insurance policy against the return of a hard border.
But it is not yet known which amendments will be chosen for a vote.
Separately, Conservative MPs on both sides of the Brexit argument have been planning for a no-deal scenario.
Speaker John Bercow is to decide which amendments are put forward, with voting taking place in the Commons from 19:00 GMT.
MPs have been tabling proposed changes to the government’s plans to try to influence the direction of Brexit since Mrs May lost the vote on her original deal earlier this month.
They include proposals to rule out leaving the EU with no deal or to delay Brexit from its scheduled date of 29 March.
The prime minister’s official spokesman said Tuesday’s voting will be followed “as soon as possible” by a second meaningful vote on whatever deal has been secured with Brussels.
But BBC political correspondent Iain Watson said Tory MPs on both sides of the argument are starting to draw up more alternatives, in case no progress can be made and the UK leaves the EU without a deal.
Former Remainers, including ex-Education Secretary Nicky Morgan and government ministers Stephen Hammond and Rob Buckland, have been working with Brexiteers Jacob Rees-Mogg and Steve Baker on the plan.
According to a leaked document, the proposal drawn up by the rival factions would extend the transition period where the UK would continue to follow EU rules and pay into its budget from the end of 2020 and into December 2021, which would allow time to reach a free trade deal.
EU citizens rights would be guaranteed during this time and there would be no customs checks on the Irish border.
Will MPs find agreement in their plans?
By Laura Kuenssberg, political editor
“It might not be 326 that matters”.
According to one cabinet minister, that’s the strange situation that Brexit has led us to.
The government’s ambition is so low – or its hurdles so high – that what No 10 seeks to do on Tuesday is not to win (326 is a majority in the House of Commons), but to reduce the scale of resistance to their central policy that, in the words of another cabinet minister, only the “hardliners oppose”, so that Theresa May can get the rebels down to a “few dozen”, so then they can crack on.
Mrs May took the deal she had negotiated with the EU to Parliament on 15 January for a “meaningful vote” – having delayed it from December – but MPs rejected it by 432 votes to 202.
She addressed a meeting of her backbench MPs on Monday night and numerous sources said she would be backing what is known as the “Brady amendment” – a measure put forward by Sir Graham Brady, the chairman of the backbench 1922 committee of Conservative MPs.
Sir Graham wants to see the Irish backstop replaced by what he calls “alternative arrangements to avoid a hard border”, but would otherwise support the prime minister’s deal.
Senior EU representatives have repeatedly ruled out reopening negotiations with the UK over Brexit, and have insisted the backstop – the insurance policy against a return of a hard border between Ireland and Northern Ireland – must be included in any deal.
And Eurosceptic Tories have already said they will not back the amendment.
Will Brussels budge on the Irish backstop?
By Katya Adler, Europe editor
Boris Johnson insists on them, Graham Brady is pushing hard for them, Prime Minister Theresa May seems to be praying for them and Dublin is deeply worried at the thought of them – but will the EU ever actually “give in” and make changes to the backstop?
It’s a tough one.
The EU certainly never intended to budge on the backstop – painfully negotiated with the UK over 18 months and signed off last November by Mrs May and her cabinet.
But Europe’s leaders didn’t imagine the UK would still be in such flux over Brexit so very close to B-day on 29 March.
Read more from Katya
The European Research Group, led by Mr Rees-Mogg, said they want the government to table its own amendment that would commit to reopening the withdrawal agreement – the part of Mrs May’s deal that lays out how the UK will leave the EU – to remove the backstop.
Mrs May has also faced calls from Labour, and a number of other MPs, to rule out the scenario in which the UK leaves the EU without a deal.
A number of Remain-backing MPs are supporting an amendment by Labour MP Yvette Cooper that would create a bill enabling Article 50 – the mechanism by which the UK leaves the EU – to be delayed by up to nine months if the government does not have a plan agreed in Parliament by the end of February.
Shadow Northern Ireland secretary Tony Lloyd told BBC Radio 4’s Today programme that Labour’s priority was to ensure that a no-deal scenario was not possible, “that Theresa May cannot be under any sense of ambiguity that she can use the tactic of saying ‘It is my deal or no deal’. No-deal must disappear.”
International Trade Secretary Liam Fox told BBC Breakfast the worst-case scenario would be no Brexit.
He said: “We have to ensure that Brexit itself is safe and we know that there are those in the House of Commons who would happily see us not leave the European Union at all.
“I think that would be potentially catastrophic in terms of our political system. But for my colleagues in the Conservative Party, I would say to them that we need to send the prime minister back to Brussels with a strong mandate, able to negotiate for the UK.”
The debate will be the first in which MPs on parental leave will be able to nominate another MP to vote on their behalf after the Commons unanimously chose to implement a year-long trial of proxy voting. Labour’s Tulip Siddiq, who delayed giving birth to attend the Brexit-deal vote on 15 January, is set to be the first person to benefit from the move.
Meanwhile, the government has announced its plans for EU citizens coming to the UK in the case of a no-deal Brexit, saying it would “seek to end free movement as soon as possible”.
The Home Office said that for a “transitional period” after Brexit – set for 29 March – EU citizens will be able to enter the UK to visit, work or study as they do now, but after three months they would need to apply for “European Temporary Leave to Remain”, which would last three years.
Home Secretary Sajid Javid said it was a “practical approach” and would “minimise disruption to ensure the UK stays open for business”.