The Spanish Prime Minister's again threatening to vote against the UK's Brexit deal at a meeting of European leaders on Sunday.
Pedro Sanchez says he and Theresa May remain "far apart" over Gibraltar’s future.
While Gibraltar is British territory, the small area remains a source of contention between the Spanish and British governments.
The draft Brexit agreement needs a majority vote on Sunday, so no single EU state can veto it.
However, opposition from a large state such as Spain would damage EU leaders' efforts to achieve a united front over Brexit.
Theresa May yesterday expressed confidence that Spain and the UK would be able to reach an agreement.
However, taking to Twitter late last night, Mr Sanchez insisted he'd continue to defend Spain's interests:
After my conversation with Theresa May, our positions remain far away. My Government will always defend the interests of Spain. If there are no changes, we will veto Brexit.
— Pedro Sánchez (@sanchezcastejon) November 22, 2018