U.K. and European Union leaders have agreed on the terms for Brexit. But that’s not the end of it. Prime Minister Theresa May needs to get the deal approved by the British Parliament, where she faces almost certain defeat. She’s not saying what she’ll do if her plan for exiting the bloc gets voted down, but that outcome would probably usher in a period of unprecedented political chaos. Here’s a guide to what comes next -- the most perilous part of Brexit.

1. What’s the deal?

It’s the most important international agreement for Britain since the end of World War II. Negotiated over 17 months, the deal sets out the terms of separation that allows the U.K. to depart the EU on March 29 in an orderly fashion — and brings with it a 21-month grace period to give everyone time to adjust. Alongside it is a political declaration that specifies that the two sides want close economic and trading ties, though the details will take years to work out.

2. What would change under the deal?

If the agreement is approved, nothing will change until January 2021, when a grace period ends. Even that could be extended for another year or two. Life in the U.K. will go on as before, with all EU rules applying -- including free movement of goods and people across borders -- but the U.K. will no longer have a say when rules are drawn up. It’s not clear yet how dramatically life will change after the transition period. That’s because the details of the future relationship are still to be hashed out. But as things stand, free movement of people is set to come to an end, and the U.K. will leave the EU’s single market. That means trade becomes trickier. EU citizens who were already in Britain before Brexit will be able to stay, and vice versa.

3. What happens next?

The deal goes to Parliament on Dec. 11 -- unless the government decides to postpone or pull the vote. There is opposition on all sides: pro-Brexit hardliners in May’s Conservative Party, pro-EU Conservatives, the Northern Irish party that’s been propping up the government and nearly all of the opposition Labour Party. The main objection is to guarantees May has offered to make sure a new physical border doesn’t emerge on the island of Ireland, which is divided between the U.K. and the Republic of Ireland, which remains in the EU. Critics say the pledges risk binding the U.K. to EU rules forever. They argue that she’s caved to the EU and betrayed the electorate’s call to regain sovereignty, while treating Northern Ireland differently from the rest of the country.

4. What happens if Parliament votes no?

Things would get murky. Labour would push for a general election, but it’s not clear the party would succeed. The ensuing chaos would provide the best opportunity for lawmakers to try to trigger a second referendum in a re-run of the June 2016 vote to leave the EU. For now, there’s not enough support for it in Parliament, but that could change. May could be ousted by her own party, she could call an election, or she could announce a host of measures for a “no-deal” Brexit and put the agreement back to Parliament a second time. The Cabinet could decide to adopt a new approach to Brexit, in an attempt to win a majority for a deal in the House of Commons. That would almost certainly mean trying to keep closer ties with the bloc. Investors are betting on May getting it through on a second vote, possibly after an adverse market reaction.

5. What’s a “no-deal” Brexit?

There’s a chance that if the deal is voted down, Britain would crash out of the bloc with no agreement on March 29. That would leave the U.K. with no legal arrangements to smooth trade and other transactions with its neighbors, snarling cross-border commerce and freezing markets. Bottlenecks could bring shortages of everything from food to drugs and manufacturing components. But that scenario is probably becoming less likely as Parliament is being increasingly assertive in trying to prevent it.

6. What will the EU Do?

May could go back to Brussels to try to extract what would probably be nothing more than token concessions. There’s a summit on Dec. 13-14 where she could make her case to EU leaders, but they have been pretty clear they don’t want to reopen the negotiation.

7. Could May get pushed out?

So far efforts to oust her as party leader and thus prime minister have failed, but there would be fresh momentum if she lost the showdown with Parliament. Rebels need 48 of the 315 Conservative lawmakers to send in letters calling for a vote, which would follow as soon as possible. A larger number of them -- 158 -- would then need to vote to replace her. That’s different to a Parliament-wide vote of no confidence in the government, which could pave the way to a general election.

To contact the reporter on this story: Emma Ross-Thomas in London at erossthomas@bloomberg.net

To contact the editors responsible for this story: Flavia Krause-Jackson at fjackson@bloomberg.net, ;Rosalind Mathieson at rmathieson3@bloomberg.net, Leah Harrison Singer, Steve Geimann

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