By Agence France-Presse
May has final day talks scheduled with EU leaders Jean-Claude Juncker and Donald Tusk, although diplomats said the withdrawal agreement is finished and ready for EU leaders to approve on Sunday.
Nothing in the painful 17-month withdrawal process has gone smoothly, and on Friday, Spain's Prime Minister Pedro Sanchez warned he might not attend if the leaders do not acknowledge that Madrid holds a veto over the fate of Gibraltar in any post-Brexit negotiation of new EU-UK ties.
Visiting Cuba, Sanchez said that Madrid must be allowed to negotiate directly with London on Gibraltar and give its specific assent to any changes to its relationship to the European Union in a future agreement between Britain and Brussels.
"If there's no agreement, it's very clear what will happen, there very probably won't be a European Council" summit, he said.
Gibraltar, a tiny rocky outcrop home to a port and around 30,000 people, is a British territory claimed by Spain and will be a bone of contention as London negotiates a new relationship with Brussels after Brexit day on March 29.
Luis Marco Aguiriano Nalda, Spain's secretary of state for European affairs, said Madrid wanted London to put in writing that it shared Madrid's interpretation of the negotiated Brexit deal texts regarding its stance on Gibraltar.
"We have demanded that it be published by the British authorities before the European Council on Sunday," said in Brussels.
In London, however, a Downing Street source said he did not know what document Aguiriano could be referring to and added: "We have negotiated on behalf of the whole of the UK family. That includes Gibraltar and the overseas territories."
In legal terms, Spain's disapproval would not halt the divorce settlement, but it would embarrass EU leaders keen to show that the 27 are united.
And, as Aguiriano noted, any final relationship negotiated between London and Brussels after Brexit day on March 29 would have to be approved by all remaining member states -- giving Spain a de facto veto further down the line.
May is due in Brussels later on Saturday to see EU Commission president Juncker, head of the bloc's executive, and EU Council president Tusk, whose institution represents the member states.
But European diplomats told AFP no more substantive negotiations would take place and Sunday's summit would simply see leaders sign off on the fruit of 17 months of dialogue.
A European diplomat said the meeting's minutes would include language stressing the importance of Britain maintaining a level playing field in trade rules during the 21-month post-Brexit transition, and on fishing rights.
And the summit will make it clear that the European Council would take the lead over the Commission in negotiating future ties -- another measure to reassure Madrid that its voice will be heard before any final settlement is reached.
After that, May will still have to sell the deal to the British Parliament, an even greater political challenge.
May refused to say whether she would resign if parliament eventually votes down the divorce agreement that the EU is set to endorse on Sunday, alongside a shorter political framework to guide talks on future ties.
"If this deal does not go through, we are back at square one. What we end up with is more division and more uncertainty," she warned.