Inviolability of Correspondence

Inviolability of correspondence is a core protection of diplomatic tradecraft, ensuring that a sending state can communicate with its mission abroad without surveillance or interference by the receiving state. It is codified in Article 27 of the Vienna Convention on Diplomatic Relations (1961), which obliges receiving states to permit and protect free communication on all official purposes, and declares that the official correspondence of the mission is inviolable.

The principle extends to several concrete instruments:

• The diplomatic bag, which under Article 27(3) must not be opened or detained, provided it bears visible external marks and contains only diplomatic documents or articles intended for official use.
• The diplomatic courier, who enjoys personal inviolability while performing functions (Article 27(5)).
• Use of codes and ciphers, expressly permitted under Article 27(1), though installation of a wireless transmitter requires the consent of the receiving state.

Parallel protections exist for consular posts under Article 35 of the Vienna Convention on Consular Relations (1963), though the consular bag regime is slightly weaker: if authorities have serious reason to believe it contains something other than official correspondence or articles, they may request it be opened in their presence, and if refused, return it to origin.

In practice, inviolability of correspondence has been tested by intelligence-era controversies, including allegations of bag tampering, electronic interception of embassy traffic, and disputes over the scale of bags (notably the 1984 Dikko affair in London, where Nigerian agents attempted to smuggle a former minister in a crate falsely labelled as diplomatic baggage — UK authorities opened it precisely because it failed the bag's legal definition).

The rule is reciprocal in character: states uphold it largely because their own missions abroad depend on the same protection.