With Britain's terms of divorce from the EU clinched on Friday, the two sides now face major hurdles to agree a transition period and future trade ties in less than a year.
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The EU wants the final version of the full withdrawal agreement ready by October 2018 to allow time for the British government and the European Parliament to approve the document.
The formal withdrawal date is set for March 29, 2019.
European Union President Donald Tusk said that in fact "the most difficult challenge is still ahead" because it is much harder to break up and build a new relationship than just to divorce.
When they meet at a December 14-15 summit in Brussels, the EU leaders must formally sign off on their negotiators' confirmation that "sufficient progress" on divorce terms have been achieved to warrant starting talks on the transition period and future trade ties.
Culminating six months of often acrimonious talks, the two sides agreed arrangements Friday for the Irish border, Britain's divorce bill, and the rights of EU citizens in Britain and British people in the bloc.
The leaders will also be asked to approve nine guidelines Tusk has sent them.
These conditions require Britain during the transition period to respect current and any new EU laws, budgetary commitments, and European judicial oversight.
Tusk noted, but did not commit to, Britain's request for a two-year transition period to reassure businesses and people.
The EU's chief Brexit negotiator Michel Barnier says the negotiations on the transition period could begin early next year, but talks on future ties would start "a bit later".
