Britain has called the opening of a diplomatic bag by the Spanish Guardia Civil at the border with Gibraltar “a serious infringement” of the Vienna Convention on Diplomatic Relations.
The incident occurred on Friday as the bag was taken from Gibraltar to Spain.
The Foreign Office said the British embassy in Madrid had made a formal diplomatic protest.
It comes a week after the UK protested about a Spanish incursion into Gibraltarian waters.
Diplomatic bags 'inviolable'
The Foreign Office would not comment about what was in the bag but a spokesman said: “Protocol concerning official correspondence and the diplomatic bag is governed by the 1961 Vienna Convention on Diplomatic Relations.
“We take very seriously any reported abuse of the protocol surrounding official correspondence and the diplomatic bag. We have asked the Spanish authorities to investigate what occurred and take action to ensure it does not happen again.”
He added: “As far as we are concerned there is no justification for this infringement of the UK's rights under the Vienna Convention. Official correspondence and diplomatic bags are inviolable.”
The Foreign Office would not comment on how the bag was being taken across the border. The Gibraltar Chronicle reported that it was “being transported by courier”.
This latest spat comes a few days after Foreign Office minister Mark Simmonds told MPs that Britain was reviewing its naval policy around Gibraltar following an illegal incursion into its territorial waters by a Spanish state vessel.
The RV Ramon Margalef was challenged by the Royal Navy after it refused to leave Gibraltar waters after 22 hours despite repeated requests.
Strict border checks
That followed a number of diplomatic rows that began in the summer after Gibraltar dropped 74 concrete blocks into the sea next to its territory it said were intended to create an artificial reef and encourage sea life to flourish.
Spain said the blocks would disrupt waters used by its fishing boats.
Shortly after, Spain imposed strict controls at the border with Gibraltar, that it said were to help prevent tobacco smuggling into its territory.
However, Britain and Gibraltar argued that the stricter checks – which had led to long delays – were politically motivated.
Following protests, the European Commission ruled that the border checks had not infringed European law, but it made a number of recommendations to both Britain and Spain to try to reduce waiting times at the border.