Intensive and secretive, the talks are a final bid to seal a new partnership agreement when Britain's transition out of the European Union ends at the end of the year.
If the sides overcome their differences, the new deal would govern everything from trade and energy to transport and fisheries. If they fail, an estimated 900 billion USD of annual bilateral trade in goods and services would be damaged from January 1 by tariffs and quotas.
An EU diplomatic source and a UK official said negotiations would continue face-to-face in Brussels on Monday following a full weekend of talks.
An update on their progress and chances of a deal was expected on Wednesday or Thursday, they added.