Spain is to forge an alliance with Argentina to present a united front against Britain in UN debates over the disputed territories of Gibraltar and the Falklands.
Spanish foreign minister Jose Manuel Garcia-Margallo is to fly to the Argentine capital Buenos Aires to meet his Argentine counterpart Hector Timerman next month. The two will discuss forming an alliance, the Spanish newspaper El Pais reported.
Argentina recently lodged a new claim at the UN Security Council over the sovereignty of the Falklands, which Britain has ruled since 1833. Gibraltar has been a British Overseas Territory since 1713.
Spanish Prime Minister Mariano Rajoy has said his country will take "all necessary measures" to defend its interests in Gibraltar.
Madrid has threatened to impose a €50 (£43.30) fee on every vehicle entering or leaving Gibraltar, and there have been long delays in crossing the border.
Madrid has also complained that an artificial reef being built by Gibraltar into the Mediterranean will block its fishing vessels.
Britain this week sent its warship HMS Westminster to the territory as part of a "long-planned" training exercise, the Ministry of Defence said.