European Union leaders are gathering to seal an agreement on Britain's departure from the bloc next year, the first time a member country will have left the 28-nation bloc.

At a summit in Brussels Sunday, the leaders are due to endorse a withdrawal agreement, which would settle Britain's divorce bill, protect the rights of citizens hit by Brexit and keep the Irish border open.

They will also rubber stamp a 26-page document laying out their hopes for future relations after Britain leaves at midnight on March 29.

The last big obstacle to a deal was overcome on Saturday, when Spain lifted its objections over Gibraltar.

The deal must still be endorsed by the British parliament and EU parliament.