Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 68. Precautions while Receiving Parlementaires
The UK Military Manual (1958) states:
All measures necessary to prevent the parlementaire from taking advantage of his mission to obtain information are allowable. Care should be taken to prevent him and his attendants from communication with anyone except the persons nominated to receive him. If permission is given for the parlementaire to enter the position for the purpose of negotiation, or if the officer in command of the position or post, or any superior officer, thinks it desirable for any special reason to send him to the rear, he should be blindfolded, and taken to the destination by a circuitous route. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 410.
The UK LOAC Manual (2004) states:
10.8. The commander to whom a parlementaire is sent is not obliged to receive him in every case. However, it is no longer permissible for a belligerent to declare beforehand, even for a stated period, that he will not receive parlementaires. However, the commander is entitled to take all steps necessary to protect the safety of his position or unit and to prevent the parlementaire from taking advantage of his visit to secure information.
10.8.1. The reason for these security measures is that there may be troop movements in progress or, owing to the state of the defences, it may be considered undesirable to allow an envoy to approach a besieged locality. Measures taken may involve prescribing the route he takes, the hour and place of his visit or even blindfolding. An unnecessary repetition of visits need not be allowed. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 10.8–10.8.1
The UK Military Manual (1958) states:
A commander has the right to detain a parlementaire temporarily if the latter abuses his position. In addition, a commander has, by a customary rule of international law, the right to retain a parlementaire so long as circumstances require, if the latter has seen anything, knowledge of which might have adverse consequences for the receiving forces, or if his departure would coincide with movements of troops whose destination or employment he might guess. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 412.
The UK LOAC Manual (2004) states:
A commander has the right to detain a parlementaire temporarily if the latter abuses his position. In addition, a commander has, by a customary rule of international law, the right to retain a parlementaire so long as circumstances require, if the latter has seen anything, knowledge of which might have adverse consequences for the receiving forces. It is not, however, an abuse of his position for the parlementaire to report back anything he may have observed. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 10.9.