Spain is accusing the European Union of "treacherous" action on Gibraltar, a disputed territory on Spain's southern coast, ahead of Sunday's Brexit signing-off summit.

Top Spanish officials are threatening to disrupt the Brexit negotiating process over Gibraltar as the talks head toward their final stages, according to multiple reports.

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Luis Marco Aguiriano Nalda, Spain’s secretary of state for the European Union, said on Thursday that a clause on Gibraltar was added in a "nocturnal and treacherous" way to the preliminary deal, Bloomberg reported. Prime Minister Pedro Sánchez is threatening to "veto Brexit" if Spain's demands on the territory are not met.

While Spain does not have unilateral veto power, it is likely that other members of the European Union would be hesitant to sign onto the Brexit deal if Spain does not support it, according to the Financial Times.

“After my conversation with [British Prime Minister] Theresa May, our positions remain far away," Sánchez tweeted on Thursday. “My government will always defend the interests of Spain. If there are no changes, we will veto Brexit.”



Madrid is seeking a guarantee that Gibraltar, a British Overseas Territory, would not be included in a U.K.-EU free trade agreement automatically. The country is demanding that Gibraltar's place in an agreement would be negotiated through bilateral talks between Spain and the U.K.

"It has to be crystal clear: the tenor of the withdrawal deal and the tenor of the political declaration have to be abundantly clear that any future relations between the EU and the UK regarding Gibraltar have to be negotiated separately from the general future relationship between the EU and the UK, and must get Spain’s seal of approval," Aguiriano Nalda said, The Guardian reported.

May has insisted that the issue can be resolved.

The EU and the U.K. this week reached a tentative deal on the future of their political relationship following Brexit. EU leaders can approve the draft deal at a summit on Sunday.

Britain is slated to leave the EU on March 29 following an internationally watched referendum.