Spain has said it will vote against the divorce agreement between the UK and the EU if there is no clarification over the future status of the handling of Gibraltar.
The UK and the EU reached a 585-page draft withdrawal agreement and are fleshing out a far less detailed seven-page declaration on their future relations.
Both need to be approved by European leaders at a weekend summit.
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If the documents do not change before Sunday, "Spain will vote no", Pedro Sanchez, The Spanish Prime Minister, has warned on Tuesday during a business forum organised by The Economist in Madrid.
Spain is the only nation among the 27 countries that has voiced opposition to the deal because it deems unclear the wording of how Gibraltar, the British territory at the southern tip of the Iberian Peninsula, would be dealt with.
Spain maintains a claim to Gibraltar after it was ceded to the British crown under the 1713 Treaty Of Utrecht, and through the Brexit negotiations Spain has conducted separate talks with the UK government over the future of the border.
However, Spain feel there's no real clarity over Theresa May's Brexit agreement on the future border relations with Gibraltar and aims to vote down the proposals.
Fabian Picardo, Gibraltar’s Chief Minister, has accused Spain of “raising issues at the eleventh hour” is a tactic Spain has used previously while the UK were in the EU.