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United Kingdom of Great Britain and Northern Ireland
Practice relating to Rule 66. Non-Hostile Contacts between the Parties to the Conflict
Section C. Definition of parlementaires
The UK Military Manual (1958) states: “The usual agents in the non-hostile intercourse of belligerent armies are known as parlementaires.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 389.
The manual also states:
A person to be regarded as a parlementaire must be authorised by one of the belligerents to enter into communication with the other and must present himself under cover of a white flag. The authorisation [for a parlementaire to enter into negotiations] should be in writing and be signed by the sending commander. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 393.
The UK LOAC Manual (2004) states:
Negotiations between belligerent commanders are normally conducted, at least in the first instance, by an intermediary, known technically as a “parlementaire”, normally operating under a flag of truce, who has been authorized in writing under the signature of the sending commander. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 10.4.