LONDON (Reuters) - Britain on Tuesday accused Madrid of a "serious infringement" of diplomatic protocol after Spanish police opened a British diplomatic bag at the border with Gibraltar, inflaming a dispute over the contested British overseas territory.
Spain lays claim to Gibraltar, a small rocky outcrop off its southern coast that it formally ceded to Britain 300 years ago, and tensions between the two countries have been running high since the summer.
Britain's Foreign Office said the incident, which occurred last Friday, was the first time a fellow European Union member and Nato ally had broken international rules by opening such a bag, which is typically used to carry official correspondence.
The last time such an incident occurred was in Zimbabwe in 2000, it said.
"We take very seriously any reported abuse of the protocol surrounding official correspondence and the diplomatic bag," the Foreign Office said in a statement. "Official correspondence and diplomatic bags are inviolable.
"We have asked the Spanish authorities to investigate what occurred and take action to ensure it does not happen again."
Opening the bag was a breach of the Vienna Convention on Diplomatic Relations, an international treaty signed in 1961 that sets out the rules on how countries should treat each other's diplomats, the statement added.
Tensions over Gibraltar flared up during the summer after Spain complained that an artificial reef being built there would block its fishing vessels.
Britain has since protested against what it said were illegal Spanish border checks and earlier this month complained about a Spanish ship carrying out what it said was unauthorized surveying activity in Gibraltar's territorial waters.
(Reporting by Peter Griffiths; Editing by Andrew Osborn)