The Foreign and Commonwealth Office (FCO) has raised "serious concerns" after a Spanish Navy vessel wrongly claimed commercial ships off Gibraltar were in Spanish waters.
Spanish Ambassador Federico Trillo has been summoned to appear at the Foreign Office following the incident, which occurred in international waters.
The FCO says the Spanish vessel's actions constitute a breach of the UN Convention on the Law of the Sea.
Minister for Europe, the Rt Hon David Lidington MP, said: “I am extremely concerned by the actions of a Spanish Navy vessel which sought to redirect two commercial vessels heading to and from the Port of Gibraltar, wrongly claiming they were in Spanish waters.
“I condemn this provocative activity and urge the Spanish government to ensure that it is not repeated. Her Majesty’s Government will continue to take whatever action we consider necessary to uphold the interests of Gibraltar, its people, its security and economy."
Prime Minister David Cameron raised Gibraltar with Spanish Prime Minister Mariano Rajoy in the margins of an EU Council meeting.
He made it clear there must not be a repeat of last summer's disruption when the imposition of additional border checks by the Spanish led to long delays at the frontier.