In the latest of a series of rows between Britain and Spain over Gibraltar, London has accused Madrid of breaching the Vienna Convention on Diplomatic Relations.
Spanish police opened a diplomatic bag at the border with Gibraltar on Friday.
Earlier, the UK protested about a Spanish incursion into Gibraltarian waters. The RV Ramon Margalef was challenged by the Royal Navy after it refused to leave Gibraltar waters after 22 hours despite repeated requests.
The BBC reports 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.''
The Gibraltar Chronicle reported that the diplomatic bag was "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.
Correspondence is also above national law and the Vienna convention also gives special status to couriers who accompany the diplomatic mail.
A BBC correspondent said the incident was "very unusual". The last occasion on which British diplomatic bags were opened was in Zimbabwe 13 years ago.
Earlier this year, Gibraltar dropped 74 concrete blocks into the sea which it said were intended to create an artificial reef and encourage sea life to flourish.
Spain said the blocks would disrupt waters used by 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, the European Commission ruled that the border checks did not infringe European law.