BRUSSELS/LONDON - Prime Minister Theresa May said she would be returning to Brussels on Saturday to hammer out an outline of Britain's future ties with the European Union after her meeting with the head of the bloc's executive on Wednesday failed to bridge the gaps.

May met European Commission head Jean-Claude Juncker for about an hour and a half to try to win commitments aimed at helping her appease rebels at home opposed to her draft Brexit treaty.

"There were some remaining issues which we have discussed," May said afterwards. "I will be returning on Saturday for further meetings, including again with President Juncker to discuss how we can ensure that we can conclude this process."

With just over four months before Britain's departure, May is trying to finalise an outline of future relations before a summit of EU leaders on Sunday due to endorse the Brexit accord.

Raising the stakes, EU diplomats said they had been told that German Chancellor Angela Merkel was not willing to come on Sunday for any more talks, meaning a text must be ready beforehand.

Spanish Prime Minister Pedro Sanchez meanwhile, insisted he would vote against the draft deal unless he wins assurances over the disputed peninsula of Gibraltar.

On Wednesday night, May spoke to Sanchez and "there had been good engagement between the UK, the Government of Spain and the Government of Gibraltar," May's office said, adding that the UK and Gibraltar "looked forward to these discussions continuing."

May said negotiators from both Britain and the European Union would continue working on the text. "I believe we have been able to give sufficient direction for them to be able to resolve those remaining issues," she said.

A spokesman for the European Commission said "very good progress" was made at May's meeting with Juncker: "Work is continuing."

Diplomats in Brussels said EU states' concerns over fishing rights and future trade ties had largely been addressed in the political declaration on the EU-UK future ties that would form a package with the legally binding divorce deal, meaning Spain's concerns were the final issue to be settled.

"The only thing really outstanding is Gibraltar," said one EU diplomat.

Sanchez has demanded assurances that the future of Gibraltar would be settled through direct talks with Madrid.

"If this is not solved by Sunday, Spain, a pro-European government, will unfortunately have to vote 'No'," Sanchez said at a news conference in Valladolid with his Portuguese counterpart