相关规则
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 68. Precautions while Receiving Parlementaires
Section B. Detention of parlementaires
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.