The deal took 17 months of tough negotiations before it was hammered out. It covers finance, citizens' rights, Northern Ireland, a transition phase, and reflects on future security- and trade ties.
The final draft was almost derailed by a last-minute row over the British territory of Gibraltar, which Spain claims as its own, which resolved on Saturday with the UK promising to bilateral talks would continue on the issue.
Theresa May said the Brexit deal would deliver her country a "brighter future".
But there is one last hurdle to overcome: the British House of Commons in London must still approve the deal before "Brexit day", designated by the European Union (Withdrawal) Act 2018 as 29 March, 2019 at 11:00 pm.
The Sunday Telegraph reported that members of May’s cabinet and EU diplomats are secretly working on "plan B" proposals, after 91 Conservative MPs indicated that they would oppose it in the Commons.
But in the letter, published in several newspapers, May insisted her agreement would "honour the result" of the 2016 referendum – when 52 percent backed Leave – and that it would be "a moment of renewal and reconciliation".
'No reason to be happy'
Until the agreement is approved, all sides are still bracing for the possibility that Britain ends its four-decade EU membership with no new arrangements in place – the so-called “hard Brexit.”
EU Council President Donald Tusk, who has always said he would prefer Britain not to leave, said on the eve of the summit that "no-one will have reasons to be happy" when Brexit happens.
But he said terms had been agreed that would "reduce the risks and losses", and recommended that EU leaders sign off on the deal.