It comes after Spanish Prime Minister Pedro Sanchez received assurances from the UK government over Gibraltar, and dropped his threat to boycott the summit, BBC reported on Saturday.

He said he had received the written guarantees he needed over Spain's role in the future of the British territory.

UK Prime Minister Theresa May has arrived in Brussels and held talks with top EU officials, ahead of the summit.

Meanwhile, former UK Foreign Secretary Boris Johnson said the UK would become a "satellite state" under the deal.

The UK is scheduled to leave the EU on March 29, 2019.

The terms of the UK's withdrawal have been under negotiation since June 2016 following a referendum in which 51.9 per cent voted to leave the EU.

Even if the EU approves the deal, it still has to be passed by the UK Parliament, with many MPs having stated their opposition.

Spain had raised last-minute objections ahead of the summit about how the issue of Gibraltar had been handled in the Brexit talks so far.

But EU leaders secured a compromise with the Spanish prime minister, who said that Europe and the UK "had accepted the conditions set down by Spain" and so would "vote in favour of Brexit".

Tusk, who represents EU leaders on the world stage, said he recommended "that we approve on Sunday the outcome of the Brexit negotiations" in a letter to members of the European Council.

He added: "No-one has reasons to be happy. But at least at this critical time, the EU 27 has passed the test of unity and solidarity."

May met the president of the European Commission, Jean-Claude Juncker, and Tusk for talks on Saturday evening.

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 39 billion pounds "divorce bill", citizens' rights and the Northern Ireland "backstop" -- a way to keep the border with the Republic of Ireland open, if trade talks stall.

There is no formal vote on Sunday but the EU expects to proceed after reaching a consensus.

If the EU signs off the withdrawal deal, May will then need to persuade MPs in the UK Parliament to back it.

A vote is expected in December. Labour, the Liberal Democrats, the SNP and the DUP have all said they will vote against the government's deal, as well as many Conservatives.

Spain threatened to derail the Brexit deal over concerns about its role in future trade arrangements involving Gibraltar -- a British Overseas Territory with 30,000 residents.

But the UK published a letter it had sent to Spain reassuring it over the withdrawal agreement.

After holding emergency talks with Tusk and Juncker, the Spanish Prime Minister dropped his threat.