Britain has formally protested to the Spanish authorities after Spanish 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 British Gibraltarian waters.
The last occasion on which British diplomatic bags were opened involved Mugabe's Zimbabwe 13 years ago, so this is the sort of company Spain is now putting itself with.
After Prime Minister's Questions in the Commons today, David Lidington, the Minister for Europe (including the British Overseas Territories of Gibraltar and Akrotiri and Dhekelia) answered an Urgent Question (UQ) from an MP about the incident.
MPs were pressurising Lidington to expel the Spanish Ambassador should the Spanish against cause trouble over Gibraltar, such as this incident or any of their ships entering British Gibraltar waters.
UK protest at Gibraltar diplomatic bag opening
BBC News
26 November 2013
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'
Gibraltareans show which side they're on
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.
"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.
Analysis
Nick Childs World Affairs Correspondent, BBC News
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.
BBC News - UK protest at Gibraltar diplomatic bag opening
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 British Gibraltarian waters.
The last occasion on which British diplomatic bags were opened involved Mugabe's Zimbabwe 13 years ago, so this is the sort of company Spain is now putting itself with.
After Prime Minister's Questions in the Commons today, David Lidington, the Minister for Europe (including the British Overseas Territories of Gibraltar and Akrotiri and Dhekelia) answered an Urgent Question (UQ) from an MP about the incident.
MPs were pressurising Lidington to expel the Spanish Ambassador should the Spanish against cause trouble over Gibraltar, such as this incident or any of their ships entering British Gibraltar waters.
UK protest at Gibraltar diplomatic bag opening
BBC News
26 November 2013
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'
Gibraltareans show which side they're on
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.
"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.
Analysis
Nick Childs World Affairs Correspondent, BBC News
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.
BBC News - UK protest at Gibraltar diplomatic bag opening
Last edited: