MPs are preparing to vote on whether to back Theresa May's deal for leaving the European Union.
The so-called "meaningful vote" will take place later as five days of debate on Brexit come to an end.
Mrs May has called for politicians to back her deal or risk "letting the British people down".
But with many of her own MPs expected to join opposition parties to vote against the deal, it is widely expected to be defeated.
Attorney General Geoffrey Cox has opened the last day of debate, with Mrs May due to close the debate with a speech from about 18:20 GMT.
He called on MPs to recognise the "value of compromise" and "opt for order [over] chaos".
Voting will start at about 19:00 GMT,
starting with votes on three or four backbench amendments that could reshape the deal and then the vote on the withdrawal agreement itself.
The prime minister addressed her cabinet on Tuesday morning before heading to the Commons for the start of the debate on her deal - which includes both the withdrawal agreement on the terms on which the UK leaves the EU and a political declaration for the future relationship.
It followed a meeting with her backbenchers on Monday night where she made one last appeal for their support before the vote.
She had also tried to reassure MPs from all sides of the House over the controversial Northern Irish "backstop" - the fallback plan to avoid any return to physical border checks between the country and Ireland - having received
new written assurances from the EU that it would be temporary and, if triggered, would last for "the shortest possible period".
Media captionBrexit plan: Political declaration and withdrawal agreement
In an impassioned speech to the Commons with the prime minister by his side, Mr Cox said voting for the deal was "the first of two keys that will unlock our future with the European Union", allowing the government to focus on the future relationship.
He said that MPs could not underestimate the complexity of Brexit and that the deal offered an "orderly, predictable and legally certain" way of leaving the bloc, without "thousands" of legal issues arising for citizens and businesses.
"If you found yourself suddenly with the rug pulled from under you, not knowing what your legal obligations would be, you would say to this House, 'What are you playing at? What are you doing?'
"You are not children in the playground, you are legislators, and it is your job. We are playing with people's lives," he said.
Media captionGeoffrey Cox: "You are not children in the playground"
But many remain opposed to the deal, with about 100 Conservative MPs and the DUP's 10 MPs expected to join Labour and the other opposition parties in voting it down.
Former Brexit secretary Dominic Raab said that Brexiteers like him could back a deal if aspects such as the backstop were dealt with.
But he told the Today programme the EU had played "a smart game of hard ball" and said it was time for the UK to do the same.
Media captionDUP's Sammy Wilson: "The deal would remove us from the rest of the United Kingdom"
Democratic Unionist Party leader Arlene Foster said the backstop "was something that we could not accept" and said her party's MPs would be voting against the deal.
"It (the backstop) does violence to the union - it separates us from the rest of the United Kingdom in a very very obvious way," she said, calling for Mrs May to "get rid" of the backstop.
And speaking during the Commons debate, the leader of the SNP at Westminster, Ian Blackford, reiterated his belief that there was "no such thing as a good Brexit" and his party would continue to fight against leaving.
"There is no way that the Scottish National Party and the people of Scotland are going to be sitting in that bus as the prime minister drives us off the cliff," he said.
EU preparations
BBC Brussels correspondent, Adam Fleming, said the European Commission President Jean-Claude Juncker will be returning to the city later to deal with "Brexit-related business", fuelling speculation Mrs May will make an emergency visit on Wednesday if her deal is voted down.
When asked by the BBC for a message to MPs ahead of the vote, he said: "I would like them to behave in a responsible way."
The EU's chief Brexit negotiator, Michel Barnier, also appealed for MPs to "keep calm" when voting later.
He will be holding talks with the European Parliament's Brexit steering group later tonight in Strasbourg before a debate on Wednesday morning about the result of the meaningful vote.
The deal suffered a heavy defeat in the House of Lords on Monday night, as peers backed a Labour motion by 321 votes to 152.
While the Lords vote carries no real weight, as peers accepted MPs should have the final say, the motion - which also rejected a "no deal" scenario - expressed "regret" that Mrs May's deal would "damage the future economic prosperity, internal security and global influence" of the UK.
However, five Conservative Brexiteer MPs who have been critics of the withdrawal agreement have now said they will support the government, along with three Labour backbenchers and independent Frank Field.
Brexit Secretary Steve Barclay said it showed there had been "progress" but admitted to the BBC's Politics Live that gaining support was "challenging".
Speaking to his own backbenchers on Monday night, Labour leader Jeremy Corbyn again condemned the deal and repeated his call for a general election if it was voted down by Parliament. He also promised Labour would call a no-confidence vote in the government "soon".
A number of amendments to Mrs May's deal were put forward - including
proposals to give MPs a vote on whether to implement the backstop and putting a time limit on it.
The Commons Speaker, John Bercow, chose four before the start of Tuesday's debate, with votes on them starting from 19:00 GMT - before the meaningful vote.
They include an amendment by the Labour frontbench rejecting the deal entirely and one from Conservative MP John Baron giving the UK the right to terminate the backstop.
For more detail on the four amendments, click here.
What happens next?
If the deal is rejected by MPs, Mrs May has three sitting days to return to Parliament with a "Plan B".
Some have suggested she would head to Brussels on Wednesday to try to get further concessions from the EU, before returning to the Commons to give a statement about her new proposal by Monday. This could then be put to a vote by MPs.
If this also fails, there is a proposal put forward by senior Conservative backbenchers Nick Boles, Sir Oliver Letwin and Nicky Morgan for a "European Union Withdrawal Number 2 Bill". This would give ministers another three weeks to come up with another plan and get it through Parliament.
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If this doesn't work either, they propose giving the responsibility of coming up with a compromise deal to the Liaison Committee - which is made up of the chairmen and chairwomen of all the Commons select committees, drawn from opposition parties as well as the Conservatives.
This proposal in turn would have to be voted through by MPs.
New referendum proposal
In another development, a cross-party group of anti-Brexit politicians has published proposed legislation to bring about another referendum to ask the public whether they want to remain in the EU or leave under the prime minister's deal.
Media captionLib Dems' Layla Moran: "Brexit is a complete cluster shambles"
The MPs behind the draft legislation point out that Article 50 - the two-year process by which an EU member can leave the bloc - would have to be extended in order for another referendum to take place, meaning the UK would remain a member beyond 29 March.
But, unless new legislation is introduced, the default position will be that the UK leaves the EU on that date with no deal.
Quick guide: What is a no-deal Brexit?
A "no-deal" Brexit is where the UK would cut all ties with the European Union overnight.
Theresa May's government, and many others, believe this would be hugely damaging and want a more gradual withdrawal. But if Parliament can't agree on that, and nothing else takes its place, the UK will leave without a deal.
This would mean the UK would not have to obey EU rules. Instead, it would need to follow World Trade Organization terms on trade. Many businesses would see new taxes on imports, exports and services, which are likely to increase their operating costs. That means the prices of some goods in UK shops could go up.
The UK would also lose the trade agreements it had with other countries as a member of the EU, all of which would need to be renegotiated alongside the new agreement with the EU itself.
Manufacturers in the UK expect to face delays in components coming across the border.
The UK would be free to set its own immigration controls. However some UK professionals working in the EU and UK expats could face uncertainty until their status was clarified. The European Commission has said that even in a no-deal scenario, UK travellers won't need a visa for short visits of up to 90 days.
The border between Northern Ireland and the Irish republic would become an external frontier for the EU with customs and immigration controls, though how and where any checks would be made is not clear.
Some Leave supporters think that leaving without a deal would be positive if the right preparations were made. They say criticism is scaremongering and any short term pain would be for long term gain.
But critics - including both Brexit supporters and opponents - say that leaving without a deal would be a disaster for the UK: driving up food prices, leading to shortages of goods and gridlock on some roads in the South East resulting from extra border checks.
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