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 will open the last day of debate at about 12:50 GMT, with Mrs May due to close the debate with a speech from about 18:30 GMT.
Voting will start at about 19:00 GMT, starting with backbench amendments that could reshape the deal and then the vote on the withdrawal agreement itself.
The prime minister addressed her backbench MPs on Monday evening in a final attempt to win support for 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.
In the Commons, she said: "It is not perfect but when the history books are written, people will look at the decision of this House and ask, 'Did we deliver on the country's vote to leave the EU, did we safeguard our economy, security or union, or did we let the British people down?'"
Mrs May also tried to reassure MPs over the controversial Northern Irish "backstop" - the fallback plan to avoid any return to physical border checks between the country and Ireland.
She pointed to new written assurances from the EU that the contingency customs arrangement being proposed would be temporary and, if triggered, would last for "the shortest possible period".
Mrs May will address her cabinet on Tuesday morning, before the debate resumes at lunchtime.
Environment Secretary Michael Gove told BBC Radio 4's Today programme that rejecting Mrs May's deal would lead to a no-deal Brexit with short term economic damage "or worse, no Brexit at all".
He said with this deal "we've picked a whole bowl of glistening cherries", despite the fact the EU had said at the beginning of negotiations that there would be no "cherry picking".
"If we don't vote for this agreement then we risk playing into the hands of those who do not want Brexit to go ahead," he said.
But many Tory MPs and the Democratic Unionists remain adamantly opposed to the deal.
About 100 Conservative MPs - and the Democratic Unionist Party's 10 MPs - could join Labour and the other opposition parties to vote 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.
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.