The exit date marks the end of the two-year negotiating period formally set in motion when the UK triggered Article 50 of the EU Treaty in March 2017. British people voted to leave the EU by 52% to 48% in a June 2016 referendum.
What have the UK and the EU agreed?
The British government and EU leaders have approved a deal on the terms of the separation — the Withdrawal Agreement — and a political declaration on the nature of future EU-UK ties.
The near 600-page Withdrawal Agreement is legally binding and settles the key “divorce” issues: the UK’s financial obligations to the EU, citizens’ rights, and arrangements for the Irish border — which will become the only land frontier between the UK and the EU. Both sides agree this must remain open, but the historically sensitive and complex issue has bedevilled the talks.
An exit deal is also needed for a planned transition period to come into effect after the UK’s departure. This is due to run until the end of December 2020, although it could be extended.
During this period many existing arrangements would remain in place, allowing for more time to sort out future EU-UK relations and avoid an abrupt “cliff-edge” exit.
The Withdrawal Agreement is accompanied by a much shorter Political Declaration on future relations. This is not legally binding — but should form the basis for a trade agreement to be negotiated after Brexit.
Signed, sealed, delivered?
Despite the EU27's endorsement of the deal on Sunday (November 25), many obstacles lie ahead on the path to an orderly UK exit from the EU next March.
The accord still needs political backing from the UK parliament and European Parliament.
During talks in Brussels, the 27 countries were united in their backing for the stance taken by Michel Barnier, the EU's chief negotiator. But the process hasn't been smooth sailing.
Until a consensus on the issue was reached on Saturday, Spain threatened to "veto" draft withdrawal deal unless there was clarification over Gibraltar, a British territory to which it maintains a claim. Meanwhile, France and some other countries called for guaranteed fishing rights in UK waters.
Now, having secured EU backing, the British government needs to get it through parliament at home and pass the necessary Brexit legislation. And staunch opposition in the House of Commons has emerged as the major stumbling block in the whole process.
The ongoing UK row over Europe
British Prime Minister Theresa May insists the deal is in the national interest and there is no viable alternative. She argues that it delivers on the referendum result: an end to free movement, an end to huge UK payments to the EU, an exit from the unpopular EU structures on farming and fishing.
However, it is a compromise and several of the original red lines laid down by Theresa May have been blurred or ditched.
Article 50 a year on: Brexit 'red lines' change colour
At the heart of the debate in the United Kingdom is the balance to be struck between two objectives: a desire for independence, sovereignty and autonomy against the need to retain access to European markets, which, the EU has always insisted, means adhering to EU rules.
Echoing the ruling Conservative Party’s decades-long internal conflict over Europe, the deal has brought hostility from both supporters and opponents of Brexit.
The UK parliament: Brexit’s ‘Rubik's Cube’
The prime minister secured the approval of her cabinet for the draft deal — but only after a long debate. In the aftermath, several ministers resigned in protest and some MPs from May's Conservative Party formally challenged her leadership.
The biggest hurdle of all is the “meaningful vote” promised to the British parliament on the deal. After several days of debate, the vote originally scheduled for December 11 was postponed the day beforehand due to the scale of the opposition. Safe passage is far from secure, and the various potentially complicated scenarios have been explored here.
May’s government commands a thin parliamentary majority thanks only to the support of Northern Ireland’s Democratic Unionist Party (DUP), who have made clear they will vote down the deal unless the so-called Irish backstop is removed.
Opposition parties have also vowed to vote against it. Labour leader Jeremy Corbyn has said it is "a bad deal which isn't in the interests of the whole country". Scottish National Party (SNP) leader Nicolas Sturgeon said it was "dead in the water". The Liberal Democrats want an “exit from Brexit” and a second referendum.
The vote has been preceded by a power struggle for control of the Brexit process. The government has suffered a series of setbacks and defeats in parliament, where MPs have sought more influence over what happens should the deal be voted down.
Eurosceptics amid ruling Conservative ranks strongly oppose May’s Brexit plan, with many calling for the UK to leave the EU with no deal. They argue the deal ties the UK too closely to EU rules, compromising independence perhaps far into the future. The European Research Group (ERG) of Tory members of parliament has several dozen members, though exactly how many would vote against a deal is uncertain.
Equally, several pro-EU MPs have also vowed to vote against a deal which, in their view, would leave the UK worse off than it had been inside the bloc. Some on both sides agree that the deal leaves the UK with no say.
Other factors are likely to come into play over a vote. The government and Labour have clashed over whether further options should be possible, beyond a binary choice between the agreement on the table and no deal. A campaign for another referendum has been growing but the potential choices, timetable, constitutional legality and political desirability of such a move have been called into question.
The prime minister has embarked upon a fierce campaign to get the deal approved, and it has been suggested that backing from business could provide added momentum.
Finally, individual politicians’ minds may be concentrated by the uncertainty and potential chaos that may result if a deal is voted down — with an eventual scenario turning out to be for them, the worst of all options.
Overall, May is likely to need the support of a significant number of opposition MPs to get a Brexit agreement through Parliament. While that is an uphill struggle, it has also been said many times that there is no parliamentary majority for “no deal” either.
January deadline and the countdown to March
By the new year, the UK government hopes to be putting forward new legislation to parliament to implement the terms agreed in a withdrawal agreement. Any later risks not allowing enough time ahead of Brexit day on March 29.
If no deal is approved, by law the government has 21 days to tell parliament how it plans to proceed.
The EU could reopen negotiations, but this would need an extension of Article 50 of the EU Treaty. The alternative would be no deal, and barely two months left for emergency planning.
A deal between the UK and EU must be passed by the European Parliament, by a simple majority of votes cast at a full session. It must also be approved by EU government leaders from 20 of the 27 countries, representing 65% of the bloc’s population. However, it does not need the approval of national parliaments.
After Brexit day
A successfully ratified deal would pave the way for an orderly UK exit from the European Union, quickly followed by full trade talks between the two sides – which were not allowed while Britain remained a member.
Under the withdrawal terms — and subject to an agreement being ratified — this is also when a 21-month transition period would begin, keeping many existing arrangements in place while allowing for more time to sort out future EU-UK relations.
If no UK-EU trade deal had been agreed by the end of December 2020 — although an extension to this period is possible under the draft accord — the famous backstop would come into force to keep the Irish border open. This would set up a basic UK-wide customs union with the EU, but with Northern Ireland more deeply integrated with the bloc's rules.
What happens in a 'no-deal scenario'?
Failure to secure formal approval from the UK and EU Parliaments will see the focus shift abruptly to “no deal” preparations, which have already been set in motion.
Many political leaders, institutions, companies and individuals have warned of severe disruption and economic damage — with the UK being hit worse than the EU. On November 28 two major reports by the UK Treasury and the Bank of England assessed the potential damage to the economy of various Brexit scenarios.
However, Brexiteers have long dismissed “Project Fear” forecasts and some argue that the UK could survive perfectly well under World Trade Organization (WTO) rules.
The UK government has published a series of papers advising citizens and businesses on the consequences and how to prepare for no deal.
The European Commission’s publication on its Brexit plans — including for no deal — warned of “significant delays” at borders and said there would be “no specific arrangement” for EU citizens living in the UK or Britons in Europe.
How did it all come to this?
Check out our history of the United Kingdom's complicated relations with the European Union in four parts: