Theresa May is heading to Brussels later ahead of a special EU summit to formally sign off the Brexit deal.
The prime minister will hold talks with top EU officials, before leaders of the remaining 27 EU countries meet to endorse the agreement on Sunday.
However, Spain has said it will not attend unless a last-minute spat over Gibraltar is sorted out.
Even if the EU approves the deal, Mrs May must still persuade enough MPs to support it, which may prove difficult.
The UK is scheduled to depart the EU on 29 March 2019.
The terms of the UK’s withdrawal have been under negotiation since June 2016 following a referendum in which 51.9% voted to leave the EU.
Arlene Foster, leader of the DUP – which Mrs May relies on for support in Parliament – will reiterate her opposition of the deal at her party conference later.
She has threatened to look again at the confidence-and-supply agreement with the Conservatives if it gets through Parliament.
However, DUP Chief Whip Jeffrey Donaldson denied the party would walk away from its agreement with the government.
But he told Radio 4’s Today programme the party was “very unhappy” with the withdrawal agreement.
He said it should be one that “everybody can support” and if voted down, he felt the PM would have to go back to Brussels and renegotiate.
Chancellor Philip Hammond told Today he was hopeful of a solution with the DUP.
He said he understood their concerns over any possible ‘backstop’, but stressed it would only be a temporary and a last resort.
The deal on offer was the “best way of Britain leaving the EU with the minimum negative impact on the economy” he added.
The chancellor said the deal was better than remaining in the EU because it honoured the referendum result and offered “the best compromise possible… satisfying both sides” of the Brexit argument.
He said Britain was being held back by “uncertainty” and the country must come “back together”.
Resurrecting ‘Project Fear’?
By Leila Nathoo, BBC political correspondent
Ministers’ hard sell of the Brexit deal continues – this morning it was Philip Hammond’s turn.
He’s been trying to reassure the DUP over their concerns about the Irish border – but he also had a message for MPs thinking of voting against the deal in the Commons.
Don’t – or chaos will be unleashed.
Warnings about the economic consequences of a Parliamentary rejection will remind some of what was dubbed ‘Project Fear’ during the referendum campaign.
The chancellor echoed Theresa May’s claim that this is the best deal on offer.
His suggestion that there is no alternative can be seen as a dismissal of those critics – the DUP, Tory Brexiteers – who think there is still a chance of renegotiation.
In fact, he went further than the prime minister, saying he believed this deal was actually better than remaining in the EU.
And that’s something that even the leading Leave campaigner and former Brexit Secretary Dominic Raab couldn’t agree with.
Meanwhile, former Northern Ireland secretary Theresa Villiers – a Leave campaigner ahead of the 2016 referendum – has said she will vote against the withdrawal agreement.
“I do not believe it is in the national interest,” she told Today.
She said “every effort should continue to be made to try and reach a better agreement” but failing that, Theresa May “should walk away”.
What will happen at the summit?
On Saturday, Mrs May will meet the president of the European Commission, Jean-Claude Juncker, and European Council President Donald Tusk for talks.
Then on Sunday, EU leaders will meet for the special Brexit summit. They will be asked to approve two key Brexit documents:
- 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
- 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 thorny issue of the Northern Ireland “backstop” – a way to keep the border with the Republic of Ireland open, if trade talks stall.
BBC News correspondent Kevin Connolly said some member states are “suspicious of the possibility the UK might be attempting some last-minute negotiations” on Saturday.
There is no formal vote on Sunday but the EU expects to proceed after reaching a consensus.
Why is Spain unhappy?
Spain has raised last-minute objections to how the issue of Gibraltar – a British Overseas Territory with 30,000 residents, 96% whom voted to remain in the EU – has been handled in the Brexit talks so far.
Spain wants more of a say over Gibraltar.
The country has long-held ambitions to bring the territory back under Spanish rule. In the face of an imminent Brexit, there’s growing concern about how their economic ties with the territory will be affected.
It wants the UK to publish a written statement promising that Spain will be directly consulted on questions relating to Gibraltar during its future trade negotiations with the EU.
Spanish Prime Minister Pedro Sanchez has said he will not decide whether to attend Sunday’s summit until the assurances are provided.
Although one country on its own cannot stop the withdrawal agreement being approved, there is “no way the EU can rubber-stamp a text when an existing member is so strongly opposed”, said BBC News Europe editor Katya Adler.
Gibraltar is believed to be the only outstanding issue ahead of the summit.
France, Denmark and the Netherlands had raised concerns over what the political declaration said about fishing rights in UK waters – but this issue is understood to have been resolved.
What happens after the summit?
If the EU signs off the withdrawal deal, Mrs May will then need to persuade MPs in her own Parliament to back it.
A vote in Parliament is expected to happen in December.
What does Theresa May say about the deal?
On Friday, the PM said the UK should not hope for a “better deal” from the EU if MPs reject her Brexit agreement.
But she declined to say whether the UK would be better off outside the EU, saying only it would be “different”.
Meanwhile, the Telegraph said it has seen leaked Cabinet papers which suggest the PM is planning to “reframe the Brexit debate around migration” – by planning restrictions on low-skilled migrants coming to the UK – in a bid to attract the support of Brexiteers.
Does the DUP’s support matter?
Yes. After the 2017 general election, Mrs May’s Conservative Party got 318 seats – four short of the number she needed to rule with a majority government.
The DUP formed a confidence and supply agreement with the Tories, promising that its 10 MPs would vote with the government, and therefore enable it to win key votes in Parliament.
The DUP opposes the Brexit deal because of the “backstop” – the last resort back-up plan to make sure a hard border between Northern Ireland and the Republic of Ireland never happens.
It will only come into effect if the UK and EU fail to agree a long-term trade deal. But the backstop would mean that Northern Ireland – but not the rest of the UK – would still follow some EU rules on things such as food products.