BARCELONA, Spain — The nation’s top diplomat said Sunday that his government is urging the European Union to side with Madrid on the future of the British territory of Gibraltar, which lies at the southern tip of the Iberian Peninsula and Spain has long sought to reclaim. Spanish Foreign Minister Alfonso Dastis told Spanish newspaper El Pais that Madrid insists it should get a veto over any agreements regarding the strategic enclave as Britain prepares to leave the EU. May’s Downing Street office said she phoned the chief minister of Gibraltar, Fabian Picardo, to assure him that Britain remains “absolutely dedicated to working with Gibraltar for the best possible outcome on Brexit and will continue to involve them fully in the process.” On Wednesday, Britain’s invocation of Article 50 of the EU’s central treaty started a two-year period to negotiate its exit for the 28-member club. Dastis said that Spain will push during Brexit negotiations for a “reciprocal principle” regarding workers’ rights and immigration.