Lawmakers are beginning five days of debate Tuesday leading up to a Dec. 11 vote on the divorce deal agreed between May's government and the European Union.
May insists the deal "delivers for the British people." But there's a strong chance Parliament will reject it, leaving Britain staring at a Brexit precipice: exit on March 29 with no deal in place.
Here's a look at what might happen if lawmakers vote down her deal.
RENEGOTIATE AND TRY AGAIN
An EU summit is scheduled for Dec. 13-14, and Britain could go back to the bloc seeking changes to the deal. EU leaders insist it is not renegotiable, but while the 585-page withdrawal agreement is locked down, the declaration on future relations is shorter and vaguer.
If that were tweaked, the government could bring the amended deal back to Parliament and hope for a different result.
But British lawmakers differ on what a better deal would look like. Many Brexiteers seek a cleaner break with the bloc, and want to change the Irish border "backstop" that would bind the U.K. in a customs union with the EU. It is very unlikely the bloc would re-open this.
Pro-EU lawmakers want a softer divorce - the so-called "Norway option" - that keeps Britain inside the EU single market for goods and services. The EU might be open to the idea, but it would mean accepting free movement of people from the bloc to Britain, a red line for many U.K. Brexit supporters.
CHANGE THE LEADER
May insists she has no plans to resign, but she may have no choice if she loses the vote by a wide margin.
Alternately, pro-Brexit Conservative rebels who have long wanted to oust her can trigger a no-confidence vote if they amass 48 letters of support. If May lost the vote, there would be a leadership contest with the winner decided by a ballot of party members - a process that would take several weeks.
May would remain prime minister in the meantime, but without much in the way of authority or a mandate, as the clock ticked down to March 29, the day Britain officially leaves the EU.
HOLD AN ELECTION
The main opposition Labour Party says it will try to trigger an election by calling a no-confidence vote in the government.
Winning it would require support from some Conservatives, who may be unwilling to support an election that could well see them ousted from power.
If May's government lost a no-confidence vote, it would have two weeks to overturn the result with a new vote by lawmakers. If that failed, there would be an election, a process that takes five to six weeks.
Whatever new government emerges would have little time to solve the Brexit conundrum before March 29.
The campaign to revisit Brexit in a second referendum - driven largely by supporters of the losing "remain" side last time around - has been gathering steam as the pitfalls and complexity of the divorce process become clear.
But the government is firmly opposed, and it's unclear whether a majority of lawmakers would back the idea in a parliamentary vote.
And what would the question 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.
There also is not enough time to hold a plebiscite before March 29, so Brexit would have to be paused - and that would require agreement from the EU.
"No deal" is the outcome almost no one wants - 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 a "no-deal" 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.
The government says stopping Brexit would betray voters' decision to leave the bloc. But some lawmakers want it to be an option, and have asked Europe's top court to rule on whether Britain could unilaterally change its mind about Brexit. On Tuesday the European Court of Justice's advocate general said that in his opinion, it could. The full court ruling is due within weeks.
ALL BETS IN PLAY
It's difficult to predict which outcome is most likely. Bookmakers certainly don't know how the Brexit process will end, though they appear to be pretty sure that May won't get her deal through Parliament next week. Ladbrokes, for example, is offering odds of 4 to 1 against her deal getting approved.
"I would not want to bet on anything," said Rob Ford, a professor of political science at the University of Manchester.
"This is a situation of what some probability scientists would call radical uncertainty. We just don't know what the relative chances of these options are because this is an unprecedented situation in lots of ways, and these outcomes are interacting, they're dynamic, they're complex. It's like a chaotic weather system."
See the AP's Brexit coverage at: https://www.apnews.com/Brexit
Copyright 2019 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.