Gibraltar's chief minister says if the United Kingdom were to vote to leave the European Union the tiny British territory on Spain's southwestern tip could find its access to the single European market blocked by a hostile Spanish government.

Fabian Picardo told The Associated Press on Sunday that Spanish Foreign Minister Jose Manuel Garcia-Margallo has warned if Britain leaves the EU, the Popular Party government would "require that we accept joint sovereignty with Spain to have access to the market."

Picardo says British Foreign Minister Philip Hammond had acknowledged that the European mechanisms in place to keep the frontier between Spain and Gibraltar flowing "will not be available to us if we are not members of the EU."

Picardo says despite the closed-border threat, Gibraltar won't cede sovereignty to Spain.