British MPs will vote on Prime Minister Theresa May's Brexit deal and a series of amendments on Tuesday.
The parliament is widely expected to vote against May's deal, opening up outcomes ranging from a disorderly divorce to reversing Brexit.
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The vote was meant to take place in December but was postponed, with May facing certain defeat. The prime minister has since sought assurances from the European Union to assuage MPs' concerns, but large numbers of her own Conservative MPs and her Northern Irish allies remain strongly opposed.
European Commission President Jean-Claude Juncker and European Council President Donald Tusk repeated in a letter on Monday that they would not reopen the divorce deal, but offered clarifications with "legal value" on the controversial "backstop" clause.
May admitted their offer fell short of what she wanted, but said MPs now had the clearest assurances they were going to get from the EU – and should support the deal.
She warned MPs of "the consequences of voting against this deal," saying a no-deal Brexit risked breaking up the United Kingdom, while the alternative of blocking Brexit "would be a subversion of our democracy."
May signed the divorce deal with other EU leaders in December after 18 months of tough negotiations, but it has faced huge opposition in parliament.
A pro-Brexit protester speaks to a taxi driver outside the Houses of Parliament in London, Britain, January 14, 2019. /VCG Photo
The main opposition Labour Party, which wants a customs union with Brussels, also dismissed the EU's assurances.
Labour leader Jeremy Corbyn said that since the postponed December vote, May had "completely and utterly failed" to get the EU to bend its position. He called the deal a "damaging shambles" that was "clearly bad for this country."
Corbyn has promised a vote of no confidence on the back of a government defeat on Tuesday. If the government loses the confidence vote, parties would have 14 days to find an alternative that had the support of a majority of MPs, or an election would be called.
Speculation is growing in Brussels and London that May could seek an extension to the Article 50 exit process if she loses the vote on Tuesday.
May on Monday said the Brexit date should not be delayed – although she did not rule out the prospect.