LONDON — Spain pushed Friday for a cast-iron guarantee of its say over the future of Gibraltar as a condition for backing a divorce agreement between Britain and European Union, as U.K. Prime Minister Theresa May battled to win approval for the deal from skeptical politicians and a Brexit-weary populace.
Spain's leader warned he would oppose the deal, which lays out the terms of Britain's departure in March and sets up a framework for future relations, if language wasn't added on Gibraltar, the disputed territory at the tip of the Iberian peninsula.
Spanish Prime Minister Pedro Sanchez — who is due to join other EU leaders at a Brussels summit on Sunday to rubber-stamp the deal — tweeted that Britain and Spain "remain far away" on the issue and "if there are no changes, we will veto Brexit."
Spain wants the future of the tiny territory, which was ceded to Britain in 1713 but is still claimed by Spain, to be a bilateral issue between Madrid and London.
Last year's EU guidelines on the Brexit negotiations effectively gave Spain veto powers over future relations between the bloc and the British overseas territory. But Spanish officials are concerned that a key clause in the agreement referring to U.K.-EU negotiations on their future relationship makes no mention of Gibraltar.
Spanish government spokeswoman Isabel Celaa said Spain required an "absolute guarantee" that any future agreement between the EU and the U.K. in matters regarding Gibraltar "will require the prior agreement of Spain."
Spain doesn't have a veto on the withdrawal agreement, which doesn't have to be approved unanimously. But it could hold up a future free-trade deal between Britain and the EU, which would require the approval of all 27 EU nations.
Spain's junior minister for the European Union, Luis Marco Aguiriano, said Friday that British authorities had made a commitment to address Spain's concerns on Gibraltar, but that he had not yet seen it in writing.
"We have a promise, a commitment, from the British government saying they are ready to ... guarantee that they will go along with the clarification we have requested," he said.
After a meeting in Brussels Friday of senior EU officials, the Spanish government said negotiations were continuing but not enough progress had been made to drop the veto threat.
Britain and the EU say the withdrawal agreement won't be changed but haven't ruled out putting something in writing to allay Spain's fears.
May said Friday that "we have been working with the government of Gibraltar and the government of Spain" on measures for Gibraltar.
Chief Minister of Gibraltar Fabian Picardo criticized Spain's insistence on a written guarantee, saying Gibraltar — a largely self-governing British overseas territory — "has demonstrated that we actually want a direct engagement with Spain on issues."
"Spain is the physical and geographical gateway to Europe for Gibraltar," Picardo told the BBC. "We recognize that and there is absolutely no need for us to be vetoed into being brought to the table."
Chancellor Angela Merkel's spokesman said Germany believed outstanding questions would be cleared up in time for Sunday's summit to go ahead.
"We assume that open questions can be cleared up by Sunday," spokesman Steffen Seibert said. "That is being worked on intensively, so the chancellor is preparing for the trip to Brussels."
If EU leaders sign off on the deal, it needs to be approved by the European and British Parliaments — a tough task for May, whose Conservatives lack a majority in the House of Commons.
May answered calls on a radio phone-in show Friday in a bid to win public support for the divorce deal, which has been slammed by pro-Brexit and pro-EU politicians alike.
Brexiteers think the agreement will leave the U.K. tied too closely to EU rules, while pro-Europeans say it will erect new barriers between Britain and the bloc — its neighbor and biggest trading partner.
May declined to say when asked by a caller whether she would resign if the deal was rejected by Parliament.
"This isn't about me," she said. "I'm not thinking about me. I'm thinking about getting a deal through that delivers for this country."
She warned that rejecting the deal would lead to "more uncertainty and more division" and could result in Britain crashing out of the bloc without agreement — an outcome feared by many businesses.
"If this deal doesn't go through what happens is, we end up back at square one," May said.
I don't think (the EU) are going to come to us and say, 'We'll give you a better deal,'" she added.
Hatton reported from Lisbon. Raf Casert in Brussels and Geir Moulson in Berlin contributed to this story.