Spain was warned by Europe minister David Lidington to halt “provocative incursions” into Gibraltarian waters after its ambassador was summoned to the Foreign Office to face British anger.
The diplomatic measure followed two “serious” incidents on Monday which saw a Spanish naval vessel patrol the area and an attempt by Spanish customs to seize a civilian boat.
Police from Gibraltar had to intervene “to safeguard the welfare of the four occupants”, the Foreign Office said of the latest incident in the long-running sovereignty dispute over The Rock.
Spain has long laid claim to the island, which is a British overseas territory.
Last month ministers made formal complaints to Madrid about “unacceptable” actions by the Spanish authorities at the border with Gibraltar which led to delays of up to six hours.
Mr Lidington launched a fresh verbal assault today after the FCO's most senior official Simon Fraser made clear Britain's concerns to Ambassador Federico Trillo.
“I condemn these provocative incursions and urge the Spanish government to ensure that they are not repeated,” Mr Lidington said.
“The UK has repeatedly made diplomatic protests to Spain over attempts by Spanish state authorities to exercise jurisdiction in British Gibraltar Territorial Waters.
“Yet on 13 November there were two further serious incursions. A large Spanish naval ship conducted a prominent patrol through British Gibraltar Territorial Waters lasting several hours.
“Later on the same day a Spanish customs vessel sought to apprehend a Gibraltarian civilian boat in British Gibraltar Territorial Waters, forcing the Royal Gibraltar Police to intervene in order to safeguard the welfare of the four occupants.”
He warned that such incidents risked damaging “the prospects of developing a harmonious and collaborative relationship between Gibraltar and Spain.
“It is also in all parties' interests to avoid incidents which could put at risk the safety of those operating in British Gibraltar Territorial Waters.
“We remain confident of UK sovereignty over British Gibraltar Territorial Waters and fully committed to protecting the interests of the people of Gibraltar and their wish to remain under British sovereignty.”