Photo/Reuters: Prime Minister Theresa May arrives in Brussels with Britain's Permanent Representative to the EU Tim Barrow
EU leaders have approved an agreement on the UK's withdrawal next March and its future relations, insisting it is the "best and only deal possible".
After 20 months of negotiations, the 27 leaders gave the deal their blessing after less than an hour's discussion.
They said the deal - which needs to be approved by the UK Parliament - paved the way for an "orderly withdrawal".
But getting the backing of MPs would be "challenging", Foreign Secretary Jeremy Hunt has told the BBC.
The UK is scheduled to leave the EU on 29 March 2019.
The EU officially endorsed the terms of the UK's withdrawal during a short meeting in Brussels, bringing to an end negotiations which began in March 2017.
European Commission President Jean-Claude Juncker said anyone in Britain who thought the bloc might offer improved terms if MPs rejected the deal would be "disappointed".
But European Council President Donald Tusk, who broke the news of the agreement on Twitter, said he would not speculate on what would happen in such a situation, saying: "I am not a fortune teller."
The UK Parliament is expected to vote on the deal in early December, but its approval is by no means guaranteed. Labour, the Lib Dems, the SNP, the DUP and many Conservatives MPs are set to vote against.
Prime Minister Theresa May has appealed to the public to get behind the agreement, arguing it is the best deal she could have struck - and honours the result of the Brexit referendum.
What has the EU decided?
The EU leaders have approved the two key Brexit documents:
- The EU withdrawal agreement: a 585-page, legally binding document setting out the terms of the UK's exit from the EU. It covers the UK's £39bn "divorce bill", citizens' rights and the Northern Ireland "backstop" - a way to keep the Irish border open, if trade talks stall
- The political declaration, which sets out what the UK and EU's relationship may be like after Brexit - outlining how things like UK-EU trade and security will work
There was no formal vote on Sunday, with the EU proceeding by consensus.
Mr Juncker said it was a "sad day" and no-one should be "raising champagne glasses" at the prospect of the UK leaving.
While it was not his place to tell MPs how to vote, he said they should bear in mind that "this is the best deal possible...this is the only deal possible".
But Lithuanian President Dalia Grybauskaite suggested there were a number of possible outcomes if the UK Parliament rejected the deal, including an extension of the negotiations, or another referendum.
What happens next?
Mrs May will now need to persuade MPs in the UK Parliament to back it.
She is expected to spend the next fortnight travelling the country trying to sell the deal before a parliamentary vote in the second week of December.
If MPs reject the deal, a number of things could happen - including leaving with no deal, an attempt to renegotiate or a general election.
Foreign Secretary Jeremy Hunt said the parliamentary arithmetic was "looking challenging" and warned "nothing could be ruled out" if Mrs May lost the vote, including the government collapsing.
He told the BBC's Andrew Marr that the UK was getting "between 70% and 80%" of what it wanted, while the agreement "mitigated" most of the negative economic impacts.
Asked if the UK would be better off than if it stayed in, he said the country would not be "significantly worse or better off but it does mean we get our independence back".
The agreement will also have to go back to the European Council, where a majority of countries (20 out of 27 states) will need to vote for it.
It will also need to be ratified by the European Parliament, in a vote expected to take place in early 2019.
What are Mrs May's critics saying?
Former Conservative leader Iain Duncan Smith said he would find it "very, very difficult" to support the agreement as it stood.
"I don't believe that, so far, this deal delivers on what the British people really voted for," he told Sky's Sophy Ridge show. "I think it has ceded too much control."
SNP leader Nicola Sturgeon - who wanted to stay in the EU - said it was a "bad deal" and Parliament should consider "better alternatives", such as remaining in the single market and customs union permanently.
And Democratic Unionist leader Arlene Foster - who wants to leave the EU - said her party's parliamentary pact with the Conservatives would be reviewed if MPs approved the deal.
She told the BBC's Andrew Marr show the agreement as it stood would leave Northern Ireland and the rest of the UK "still within European structures with no say in its rules".
Former PM Tony Blair, who backs another referendum, said the deal was "a dodo" while shadow justice secretary Richard Burgon said it was an "ill-fated, half baked deal that was the worst of all worlds".