Image copyright Reuters Image caption The row is the latest in a series of disagreements between the UK and Spain over Gibraltar

Britain has formally protested to the Spanish authorities after police opened a diplomatic bag at the border with Gibraltar.

The Foreign Office said diplomatic bags were "inviolable" and opening one was "a serious infringement" of the Vienna Convention on diplomatic relations.

The incident occurred on Friday as the bag was taken from Gibraltar to Spain.

It comes a week after the UK protested about a Spanish incursion into Gibraltarian waters.

Diplomatic bags 'inviolable'

The Foreign Office would not say what was in the bag opened by the Spanish Guardia Civil but said: "Protocol concerning official correspondence and the diplomatic bag is governed by the 1961 Vienna Convention on Diplomatic Relations.

Analysis This is a very unusual incident. The last occasion on which British diplomatic bags were opened involved Zimbabwe 13 years ago. The Foreign Office says it has no record of a previous such incident involving an EU partner or NATO ally. The 1961 Vienna convention on diplomatic relations, which codifies the status of the diplomatic bag, is one of the most widely-adopted international treaties, having been ratified by 189 countries so far. It states simply that a diplomatic bag shall not be opened or detained, but also that it must be clearly marked and contain only diplomatic documents or articles intended for official use. There have been incidents of the abuse of the diplomatic bag, for example to smuggle drugs or even people. What exactly lies behind this incident remains unclear. Was it, perhaps, a mistake? But against the background of recent Anglo-Spanish tensions over Gibraltar, it could represent a serious new development.

"We take very seriously any reported abuse of the protocol...

"We have asked the Spanish authorities to investigate what occurred and take action to ensure it does not happen again."

A spokesman 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 Gibraltar Chronicle reported that the diplomatic bag had been "being transported by courier", but the Foreign Office would not comment on this.

According to the international convention, packages carrying official documents and other material deemed necessary for use by a diplomatic mission - provided that they are clearly marked as such - cannot be opened or detained.

Just as diplomats and their embassies are exempt from the rules and regulations which govern others in a host nation, their correspondence is also above national law.

The Vienna convention also gives special status to couriers who accompany the diplomatic mail.

The BBC's world affairs correspondent Nick Childs said the incident was "very unusual".

The last occasion on which British diplomatic bags were opened involved Zimbabwe 13 years ago.

The Foreign Office said it had no record of a previous such incident involving an EU partner or Nato ally.

Strict border checks

Image copyright Getty Images Image caption The European Commission ruled that border checks had not broken EU laws

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.

That followed a number of diplomatic rows that began in the summer after Gibraltar dropped 74 concrete blocks into the sea into disputed waters just off its coast. It said the blocks 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 afterwards, 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.