Gibraltar, a rocky speck of British soil clinging to Spain's southern tip trumped fishing and even trade rules on Friday to become the last sticking point in preparations to endorse Britain's EU exit deal.

As senior diplomats from European Union member states met Friday to put the finishing touches to plans for Sunday's summit to give the Brexit divorce papers the nod, "The Rock" loomed over their efforts.

Spain urged Britain Friday to put in writing a promise that it agrees Madrid will have a veto over any future deal on relations between the EU and Gibraltar. Prime Minister Pedro Sanchez would not decide whether to attend a Brexit deal signing summit until he had seen the promise, a senior official told reporters. "We have demanded that it be published by the British authorities before the European Council on Sunday," said state secretary Luis Marco Aguiriano Nalda. Spanish PM Sanchez has threatened to "veto Brexit" if it is not made clear that Madrid and London must resolve their differences over Gibraltar before any eventual future partnership bilaterally.

In Madrid's view, neither the 585-page withdrawal deal nor the accompanying 27-page framework for future ties guarantees this, so diplomats are trying to negotiate another text to clarify the issue.

"Gibraltar is the only open issue," a European diplomat said as the so-called "Sherpas" met in Brussels to finalize preparations for Sunday's summit. "We are working hard," chief EU negotiator Michel Barnier said, as his team attempted to hammer out an annex to the summit texts with the Spanish delegation.

In strictly legal terms, Spain's objection would not halt the deal being endorsed on Sunday, but politically it would be a blow if a major member state like Spain prevents the EU from demonstrating consensus. And if Spain's concerns are not addressed in the months to come, then Madrid could still veto the adoption of any future relationship agreement that London and Brussels manage to negotiate after Brexit.

In April 2017, when Brussels and London began the marathon task of negotiating the terms of Britain's departure under Article 50 of the EU treaty, the member states endorsed very clear guidelines. "No agreement between the EU and the United Kingdom may apply to the territory of Gibraltar without the agreement between the Kingdom of Spain and the United Kingdom," it said.

Now, however, the formal withdrawal agreement only says that the EU and the United Kingdom "took note" of the April guidelines, where Spain's right to veto appears, in order to conclude the divorce. Spain does not think that this is enough, but member states and EU officials have refused to reopen the summit texts, preferring to seek an annex that would address Spain's concerns.