Tensions between the UK and Spain flared in July 2013 when Gibraltar boats dumped blocks of concrete in disputed waters used by Spanish fishermen.
Gibraltar said it was creating an artificial reef that would boost fish populations and Spain complained the blocks threaten the livelihoods of Spanish fishermen.
Spain also blamed Gibraltar for an increase in tobacco smuggling to the Spanish mainland.
Madrid subsequently stepped up border checks, leading to huge queues for motorists, in what Britain and Gibraltar charged were a retaliatory measure aimed at choking the territory's economy.
Spain ceded Gibraltar to Britain in 1713 but has long argued that it should be returned to Spanish sovereignty. London says it will not do so against the wishes of Gibraltarians, who are staunchly pro-British.