While controversy over a draft Brexit deal with the European Union continues to dominate the U.K.'s political agenda, the thorny subject of Gibraltar is upsetting Spain once again.

Spain's Prime Minister Pedro Sanchez said Spain will reject the draft Brexit withdrawal agreement, struck between the U.K. and EU last week, without a clarification of the text the will shape future talks on the status of Gibraltar — a tiny overseas territory with British sovereignty on the south coast of Spain.

The draft Brexit deal, or the "Withdrawal Agreement" as it's officially known, covers a multitude of elements regarding the U.K.'s withdrawal from the EU, including citizens' rights, the financial settlement and a transition period (of 21 months after March 2019 when Britain is scheduled to leave the EU) as well as protocols on Ireland, Gibraltar and Cyprus.

Spain wants to make sure it is not left out of any future EU-U.K. talks on Gibraltar. Sanchez tweeted Tuesday that any talks on Gibraltar's status, after the transition period ends, must be held between Spain and the U.K.

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It said these changes would have to be made on Sunday, when the European Council (the EU heads of state) meets for a special summit on Brexit.

Dominated by the Rock of Gibraltar, Gibraltar was ceded to the Brits in the early 1700s after being captured from Spain. The sovereignty of the territory has been a bone of contention between the U.K. and Spain ever since with various attempts by the Spanish to re-take the land.

Spain asserts a claim to Gibraltar although residents have rejected that in two separate referendums, the last one being in 2002. Then, the U.K. government proposed sharing sovereignty with Spain but, giving the people of Gibraltar a vote, 98.9 percent rejected the proposal.