It looks very likely that Britain's Parliament will reject the agreement in a vote scheduled for Tuesday. If that happens, May has until the start of next week to come back to Parliament with a Plan B — and Britain has just 10 weeks until it is due to leave the bloc, with or without a deal, on March 29.
Here's a look at what might happen if lawmakers vote down the deal.
SECOND TIME LUCKY
If the deal is rejected by a narrow margin, the government could try again, re-submitting the agreement to Parliament after tweaking it or winning some soothing words from the EU to assuage lawmakers' concerns.
But while EU leaders have offered "clarifications" to the deal, they insist the 585-page withdrawal agreement cannot be reopened.
Any further assurances they give are likely to fall far short of demands by pro-Brexit lawmakers, who want to alter or drop the "backstop," a guarantee for the Irish border that would bind the U.K. in a customs union with the EU after Brexit. The EU says that there can be no deal without the backstop to guarantee an open border along the U.K.'s only land frontier with an EU member state.
The EU would likely be more receptive to a softer Brexit deal that saw Britain remain part of the bloc's single market for goods and services. But May says she will never agree to such a plan and it's unclear whether a majority of lawmakers would support it.
DELAY BRITAIN'S EXIT
With Parliament split, there is increasing speculation that Britain might seek an extension to the two-year exit process that is due to expire on March 29. Even if a withdrawal deal is approved soon, there may not be enough time for Parliament to pass all the legislation that needs to be in place by Brexit day.
Some ministers are urging May to delay Brexit and then consult lawmakers in a series of "indicative votes" to see if a majority can be found for a new plan. And various factions of lawmakers are exploring ways to use parliamentary rules to wrest control of the Brexit process from the government.
A delay would likely also be needed in the event of two other possible scenarios: a general election, or a second referendum. Any delay to the date of Brexit would require unanimous approval from leaders of the EU's remaining 27 member states.
HOLD AN ELECTION
If the deal is defeated by a big margin, Prime Minister Theresa May will face pressure to resign. But she has vowed to carry on, and there is no way her Conservative Party can evict her as leader — after a failed no-confidence vote in May's leadership by Conservative lawmakers in December, she is safe from another challenge for a year.
The main opposition Labour Party says it will try to trigger an election by calling a no-confidence vote in the whole government. But the vote will fail unless some members of the governing Conservatives or the government's allies from the Democratic Unionist Party in Northern Ireland rebel and side with the opposition.
If the government lost a no-confidence vote, it would have 14 days to overturn the result by winning lawmakers' confidence in a new vote — possibly with a new prime minster, if May was persuaded to quit. Barring that, there would be an election, a process that takes five to six weeks.
The campaign to revisit Brexit in a second referendum — driven largely by supporters of the losing "remain" side in the 2016 referendum — has been gathering steam as the pitfalls and complexity of the divorce process become clear.
The government is firmly opposed, but has warned that it is increasingly likely that the decision to leave the EU could be reversed if May's deal is rejected.
It's unclear whether a majority of legislators would support a new referendum, or what the question would be. Many pro-EU politicians want a choice between leaving on the proposed terms and staying in the EU, but others say leaving without a deal should also be an option.
There's a strong chance any new referendum would be as divisive as the first.
"No deal" is the outcome few want — but it is also the default option. If the divorce deal is not approved, altered or put on hold, Britain will cease to be an EU member at 11 p.m. London time on March 29.
The Bank of England has warned that tumbling out of the bloc with no deal to soften the exit could plunge Britain into its deepest recession in nearly a century, and businesses warn the sudden end to longstanding trading agreements with the EU could see gridlock at British ports and shortages of food and medicines.
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