Spain has long laid claim to Gibraltar and the colony on the southern tip of the Iberian peninsula is the source of occasional diplomatic friction between Madrid and London.
The latest spat involves an artificial reef being built in Gibraltar that Spain says is hurting its fishermen. It has floated the idea of charging people entering and leaving Gibraltar 50 euros ($66), with the proceeds going toward compensating fishermen whose work allegedly has been affected.
Gibraltar Chief Minister Fabian Picardo told BBC radio that such fees would violate European Union freedom of movement rules, and said "hell would freeze over" before the reef would be removed.
"What we have seen this weekend is saber-rattling of the sort that we haven't seen for some time," Picardo said, describing threats of border fees as "more reminiscent of the type of statement you'd hear from North Korea than from an EU partner."
Under Spain's former Socialist government, relations between Madrid, London and Gibraltar eased greatly.
But in an interview published Sunday in Spanish newspaper ABC, Spanish Foreign Minister Jose Manuel Garcia-Margallo said the "playtime" of that era was over.
Britain's Foreign Office said it was seeking an explanation from Spain about reports it might target Gibraltar with further measures.
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