Brexit process could lead to widely different outcomes

What are the possible scenarios?

Theresa May's Deal

If Theresa May can convince first her Cabinet and then the House of Commons to back her deal, and it is also approved by the leaders of the other 27 EU nations and the European Parliament, the UK will take the orderly route to departure on March 29 2019.

This would secure the rights of EU and British expats and trigger payment of a UK financial settlement expected to reach up to £39bn (€45bn) over many years.

Brexit day would be followed by a 21-month transition period until December 2020, during which negotiations would take place on future trade relations.

If no deal can be reached in that timeframe - regarded as very tight by many experts - it is thought the UK would stay in a customs union "backstop", tied to EU rules and regulations and unable to strike its own trade deals.

This would continue until a permanent trade deal is struck or a review mechanism is used to tear the whole arrangement up.

No deal

If Mrs May's deal is rejected by the Cabinet, MPs or the EU, and no alternative arrangement can be found by March 29, the UK could leave without a withdrawal agreement.

Previous bargains on expats' rights, the transition period and the financial settlement would lapse - with some Brexiteers arguing that the UK could withhold its payments.

Intensive work would take place over the coming four months to soften the impact of a 'no-deal' Brexit on the UK's ports and airports, deliveries of food and medicines and the "just-in-time" supply chains on which major industries depend.

The UK would fall back on World Trade Organisation terms, with tariffs for many imports and exports, until it could negotiate new deals with countries round the world.

Some businesses could activate plans to move activities out of the UK, while others would be on the hunt for new opportunities.

Britain would be free to strip back 40 years' worth of EU regulations in areas like environmental and workplace protections.

A Second Referendum

There are increasingly vociferous calls for any deal - or a no-deal outcome - to be put to a "People's Vote" referendum for approval by the public.

Organising a national plebiscite at such short notice would probably require delaying the official date of Brexit.

A fresh vote is firmly opposed by the British Prime Minister, and there would be a bitter fight over the wording of the question in any referendum.

Opponents of Brexit believe that this is the most likely route to prevent EU withdrawal, with opinion polls suggesting that a majority of voters now think the UK was wrong to back Leave.

But critics say it would reignite the savage battles of the 2016 campaign, dividing the nation even more deeply.

A General Election

This is Labour's preferred option, but would depend on a particular set of circumstances.

A vote of no confidence backed by more than half of MPs would force an early poll, but this scenario requires the DUP or substantial numbers of Tories to turn against Theresa May and no other leader being able to form an alternative government within 14 days.

If Mrs May's proposals are rejected, she could call a "back me or sack me" election with an appeal to voters to help her push the plan through, but she would need a two-thirds majority in the Commons, which she may struggle to achieve.

Polls currently put Tories in the lead over Labour, but in the circumstances of a chaotic run-up to Brexit it is anyone's guess which party would prevail or whether voters would deliver another inconclusive result.

A New Conservative Leader

Mrs May is vulnerable to a vote of no confidence in her leadership if 48 Tory MPs request it.

To survive, she would need the support of 158 of her MPs, though even if she reaches that number she could be fatally wounded.

Much would then depend on the leader then chosen by party members from a shortlist of two emerging from votes by the parliamentary party.

A hardline Brexiteer like Boris Johnson or Jacob Rees-Mogg could take the UK out of the EU without a deal, while a moderate like Sajid Javid or Jeremy Hunt would stress the need to restore party unity.

In the highly unlikely scenario of a Remainer like Justine Greening taking the top job, the UK could take an entirely new approach to Brexit, drawing back from Mrs May's "red lines".

Extend Article 50

Under Article 50 of the EU treaties, the UK automatically leaves on March 29 - two years after formally notifying its intention to quit - whether a deal is reached or not.

Some critics of Mrs May's deal have called on her to ask for an extension of the period for a year or more to allow time for a better agreement to be reached and for business to prepare.

This would require unanimous approval from the EU27, which is far from certain to be forthcoming, particularly as it might take the date of Brexit past the 2019 European Parliament elections, the formation of a new European Commission and the beginning of the next seven-year Budget.

Break-Up of the Union

Senior figures in Northern Ireland have made clear that they regard the continuation of the United Kingdom itself as being under threat.

DUP chief whip Jeffrey Donaldson has said that many believe Mrs May's deal would lead to the break-up of the UK, and Sinn Féin has said it will call for a border poll in the case of a no-deal outcome.

Many backers of independence for Scotland - which voted Remain in 2016 - expect a surge of support if Brexit causes disruption and economic damage.

PA