Britain reacted angrily after Spain opened a diplomatic bag at the border with Gibraltar, contravening the Vienna Convention.
The Gibraltar Chronicle has reported that Spanish officers opened the UK government bag last Friday as it was being couriered across the border from the British overseas territory and into Spain. The bag was possibly on route to the British embassy stationed in Madrid.
"This was a serious infringement of the Vienna Convention on Diplomatic Relations," a spokesman for the FCO in London declared.
"We take very seriously any reported abuse of the protocol surrounding use of the diplomatic bag."
The incident is the latest spat in an increasingly acrimonious clash between the UK and Spain over the island of Gibraltar.
"We have asked the Spanish authorities to investigate what occurred and take action to ensure it does not happen again," the spokesman continued.
"As far as we are concerned there is no justification for this infringement of the UK's rights under the Vienna Convention. Diplomatic bags are inviolable."
She added: "There are long-established and internationally-accepted Articles concerning use of the diplomatic bag to which the FCO adheres. We expect other parties to the Convention, including EU and NATO allies, to do the same."
The dispute comes after a series incidents. Last week, Britain summoned the Spanish Ambassador to the UK, Federico Trillo, to the Foreign Office after a Spanish vessel incurred Gibraltarian waters.
The UK Foreign Secretary, William Hague, described the vessel incident as the "latest unacceptable incursion into British Gibraltar territorial waters".
Other disputes have included the strict security checks at the Spanish border which created queues of up to eight hours for Gibraltarians, while Gibraltar has placed concrete blocks on the seabed around the territory to prevent perceived overfishing by Spanish boats.