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.

Spain's prime minister, meanwhile, insisted he would vote against the draft deal unless he wins assurances over the disputed peninsula of Gibraltar.

May said negotiators from both sides 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 on Wednesday.

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.

Spanish Prime Minister Pedro Sanchez has demanded assurances that the future of the disputed British territory of Gibraltar would be settled through direct talks with Madrid.

The British and European parliaments must ratify the tentative deal to end more than 40 years of partnership before Brexit day on March 29, 2019, or Britain would be set to crash out of the EU with no treaty.

The publication last week of a draft exit treaty sparked the biggest crisis of May's premiership, with two cabinet ministers quitting and dozens of Conservative members of parliament calling on her to step down. They accuse May of making too many concessions to the EU, while others oppose Brexit altogether.

May hopes the blueprint on future relations -- a political document of about 20 pages meant to be agreed side-by-side with the legally binding 600-page exit treaty -- will help win back enough support at home for her approach to pass in parliament.

Despite the political turmoil around her, May has shown no sign in public of any doubt that she can bring home a deal.

The EU is trying to discourage Britain from any renegotiation of the draft treaty, while also seeking to assuage concerns expressed by some of the remaining 27 member states.

While unanimity is not legally required to endorse the deal now, the bloc seeks a unified stance.

Australian Associated Press