The future of Gibraltar is an issue that could derail British Prime Minister Theresa May’s Brexit deal, which has so far been agreed by EU and UK negotiators.

As reported by the Guardian, Madrid has insisted from the start of the Brexit negotiations that it would not tolerate the Rock, a disputed territory, benefiting from agreements made in the talks without Spain’s consent.

Marco Aquarian, Spain’s secretary of state for the EU, warned on November 22 that his government could “stop the clock” on the negotiations, and force May and the other EU leaders to come back in December unless it gets its way in the next 48 hours.

“We’re worried because this paragraph, which was introduced almost treacherously and under the cover of darkness, could be used by the UK in the future to argue that a future agreement between the EU and the UK could be applied to Gibraltar without necessarily requiring the prior agreement of Spain,” said Aguiriano.

According to the Guardian, while Spain does not have a veto on the Brexit deal, the 26 other EU member states would not want to adopt the withdrawal agreement and political declaration, published on November 22, without Madrid’s support.

In a separate report, Spain’s daily El País noted that Spanish PM Pedro Sánchez telephoned Theresa May to say that “the problem affects the essence of our country, of our nation”.

A spokesperson from the British government said that May used the call with Sánchez to reiterate “her commitment to agreeing a deal that works for the whole UK family including Gibraltar, the other UK Overseas Territories and the Crown Dependencies.”

The preliminary agreement, which is part of the Gibraltar Protocol in the draft Brexit deal, comprises four memorandums of understanding (MoUs) and one tax treaty, according to Gibraltar Chief Minister Fabian Picardo. Government sources have confirmed this information to El País.