A still image from a video footage shows Britain's Prime Minister Theresa May speaking during Prime Minister's Questions in the House of Commons, in central London, Britain November 14, 2018. Parbul TV | Reuters TV

On Tuesday, U.K. lawmakers will vote on whether to accept Prime Minister Theresa May's plan to leave the European Union (EU). The outcome is uncertain, particularly if May suffers defeat. Some speculate it could lead to a change of prime minister, an attempt to revise the Brexit deal, the complete collapse of government, or even a second vote on whether Britain and Northern Ireland should leave Europe at all.

What is the vote?

May's proposal is split into a "Withdrawal Agreement," setting out the terms of the divorce and a "Future Relationship" document, which drafts how the U.K. will interact with the EU in the future. The U.K. government and EU heads of state have signed off on the preliminary deal and now the British Parliament will undertake a "meaningful vote" to decide whether it can proceed.

Lawmakers will vote in the House of Commons at some point after 7 p.m. local time on Tuesday, December 11. The government's motion to Parliament is being intensely scrutinized before the vote takes place, with at least five full days of parliamentary time allotted to a Brexit debate. Should it pass the House of Commons, the proposal will also need to satisfy Parliament's upper chamber, the House of Lords.

What are its chances of success?

May faces a tough task. Her deal with Europe is seen by some as a sell-out to the ideals of Brexit, reducing Britain's influence while staying within many of the EU's rules. Many of those who oppose Brexit don't like the deal either. They argue that it will reduce Britain's ease of trade with the world, repel global talent, and increase the cost of living. May has claimed it is the best deal she could get to satisfy a deeply divided country which in June 2016 voted 52 percent to leave and 48 percent to remain in the EU.

Both pro-EU Remainers and Brexiteers protest their ideals outside the House of Commons, on 4th December 2018, in London, England. Richard Baker | In Pictures | Getty Images

Her proposal now needs the backing of 320 MPs, more than half of the 639 MPs that vote in Parliament, to pass. But the numbers look tricky. After stacking up opposing votes from the main opposition Labour party along with those from the Scottish National Party and Northern Ireland's DUP, May is already under pressure. Add to that the opposition from both the pro and anti-EU wings of her own Conservative Party and her Brexit deal looks in real trouble. One estimate in The Guardian newspaper has the prime minister suffering a huge defeat of almost 200 votes. Others predict that May is still likely to lose but by a much smaller number. Any lawmakers, political analysts or media commentators who do see the deal passing are keeping quiet.

What happens if May is defeated?

Assuming the deal is voted down, the margin of defeat is important. Should May lose by around 30, 40 or perhaps even 50 votes then it's expected she could stand up in Parliament and say she will return to Brussels and attempt to tweak the deal. If slight concessions were agreed with the EU, that could be enough to persuade some U.K. lawmakers to side with May in a second parliamentary vote. One caveat to that scenario is that the rest of the EU have shown little appetite to reopen talks. European Commission President Jean-Claude Juncker has warned the U.K. that May's Brexit deal "is the only deal possible." Should May lose heavily, then chaos is once again likely to descend upon British politics. The range of outcomes runs from a leadership challenge, a May resignation, a general election in the U.K., a second referendum on EU membership, a temporary stop on Britain's withdrawal, or some sort of combination of all the above.

Could Britain still crash out of Europe?

May, in a bid to rally support, has warned that if her deal is rejected then the options are either "No Brexit" which would fail to honor the 2016 referendum, or a "No Deal," which would see Britain completely cut ties with Europe and likely default to World Trade Organization rules. Several analyses have warned of the extreme economic damage that leaving Europe without any deal could bring. The Bank of England's recent report said if such a scenario were agreed, then unemployment could rise to 7.5 percent, house prices could fall 30 percent, sterling might crash, and the economy could shrink by around 8 percent over the course of a year.

Mark Carney, governor of the Bank of England (BOE), gestures while speaking during the bank's quarterly inflation report news conference in the City of London, U.K., on Thursday, Aug. 2, 2018. Simon Dawson | Bloomberg | Getty Images