The Corps of King's Messengers (or Corps of Queen's Messengers during the reign of a female monarch) are couriers employed by the British Foreign, Commonwealth and Development Office (FCDO). They hand-carry secret and important documents to British embassies, high commissions, and consulates around the world. Many King’s Messengers were retired Army personnel. Messengers generally travel in plain clothes in business class on scheduled airlines with their consignment. The division's official name changes depending on the gender of the reigning monarch.

The safe passage of diplomatic baggage is guaranteed by Articles 27 and 36 of the Vienna Convention on Diplomatic Relations, and for reasons of state secrecy, the diplomatic bag does not go through normal airport baggage checks and must not be opened, X-rayed, weighed, or otherwise investigated by customs of a foreign state, airline security staff or anyone else. The only exception is if there is serious evidence that it might contain materials prohibited or controlled by quarantine in the destination state.^: 8, 11 The King’s Messenger has the status of a diplomatic courier and cannot be detained by foreign governments. British diplomatic bags and King’s Messengers do not benefit from these immunities in the UK, and HMRC has the right to examine UK diplomatic bags. The messenger and the messenger's personal luggage can be required to go through normal security screening.