Barely two-and-a-half months before Britain is due to leave the European Union, May's expected defeat would open a range of outcomes, from resurrecting her deal to leaving the EU with no deal, or even another referendum that could halt Brexit
Prime Minister Theresa May faced the prospect of a historic defeat in a vote on her Brexit deal in parliament on Tuesday, plunging plans for Britain's biggest political and economic change for decades into deeper uncertainty.
Barely two-and-a-half months before Britain is due to leave the European Union, May's expected defeat would open a range of outcomes, from resurrecting her deal to leaving the EU with no deal, or even another referendum that could halt Brexit.
The prime minister's hopes of keeping her widely unpopular plan alive, and staying in her job, will hinge on the scale of her expected loss: defeat by a huge margin would make it harder to take the plan back to parliament for another vote.
"The responsibility on each and every one of us at this moment is profound, for this is a historic decision that will set the future of our country for generations," she said in a final plea to lawmakers to support her deal as Britain's deepest political crisis for at least half a century built to a head.
"I believe we have a duty to deliver on the democratic decision of the British people and do so in a way that brings our country together."
A running tally compiled by Reuters of lawmakers (MPs) who spoke in the five-day debate ahead of the vote indicated 60 would support May's deal while 165 would not. There are a total of 650 lawmakers in parliament.
A spokesman for May said the prime minister would "respond quickly to the result" of the vote. Finance minister Philip Hammond, business minister Greg Clark and Brexit minister Stephen Barclay are due to hold a conference call with business leaders at 2100 GMT, two sources told Reuters.
The unexpected withdrawal of three amendments that had been submitted for a vote meant that the crucial vote on May's deal was likely to take place earlier than expected - at around 7:30pm (1930 GMT).
May says accepting her deal is the only way parliament can fulfil its promise to deliver on the 2016 vote to quit the EU, and failing to do so would be "catastrophic" for democracy. Her ministers pressed that message.
Many Brexit-supporting lawmakers from May's Conservative Party reject the deal, particularly a "backstop" requirement that the British province of Northern Ireland stay closely aligned to EU rules to avoid the return of a hard border with EU member Ireland.
At a crossroads
Dominic Raab, who resigned as May's Brexit minister in November in protest at her plans, said the government should not lose its nerve over the possibility of a no-deal Brexit, something many employers fear would mean chaos for business.
But many lawmakers said they would do their utmost to block an exit without a deal. Sterling hit a two-month high against the US dollar on Monday as markets now expect parliament to block a no-deal Brexit, but slipped back on Tuesday and volatility levels spiked.
While the country is divided over EU membership, most agree that the world's fifth largest economy is at a crossroads and that its choices over Brexit will shape the prosperity of future generations.
May reached the agreement with EU leaders in November, envisioning continued close ties with the bloc. The EU has repeatedly said it is not up for meaningful renegotiation.
Many in May's Conservative Party say the prime minister will nevertheless probably try to win new concessions from the EU and seek another vote if she loses, even by a hefty margin.
Germany's foreign minister said the EU could hold new talks with Britain after a defeat in parliament of the plan. But he ruled out significant changes.
May has warned pro-Brexit lawmakers that if her plan is rejected, no Brexit at all would be more likely than Britain leaving the EU without a deal. She has also warned Conservatives against allowing the left-wing opposition Labour Party to seize the initiative.
Labour opposes May's deal but is formally committed to Brexit, despite many party members calling for a new referendum.
Labour leader Jeremy Corbyn, hoping to force a national election, is expected to call for a parliamentary vote of no confidence in May's government if she loses Tuesday's vote.
"The people need to be able to take back control, and a general election would give them the opportunity to decide who their MPs were, who their government was and who was negotiating on their behalf. It would give that new government a mandate ... that is needed to break the deadlock," Corbyn said.
"I say this to our negotiating partners in the EU: If parliament votes down this deal, then re-opening negotiations should not, and cannot, be ruled out."