Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 69. Loss of Inviolability of Parlementaires
The UK Military Manual (1958) provides: “If signalled or ordered to retire, [a parlementaire] must do so at once. If he does not do so within reasonable time he loses his inviolability.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 405.
The manual further states: “A parlementaire loses his right of inviolability if it is proved beyond any doubt that he has taken advantage of his privileged position to provoke or commit an act of treachery.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 413.
The UK LOAC Manual (2004) states:
10.10. A parlementaire loses his right of inviolability altogether if it is proved beyond doubt that he has taken advantage of his privileged position to provoke or commit “an act of treason.” That includes engaging in sabotage or espionage or inducing members of the enemy armed forces to desert. Any measures taken against a parlementaire or his party should be reported at once to the enemy.
10.10.1. It is forbidden to make improper use of a flag of truce. Thus, a feigned intention to negotiate or surrender with the intention of using the white flag as cover for the collection of information might amount to the war crime of perfidy whatever the consequences. It would amount to a grave breach of Additional Protocol I if it resulted in death or serious injury. A parlementaire who abuses his position in this way can be taken as a prisoner of war and tried.
Orders to withdraw
10.11. If ordered to withdraw, the parlementaire must do so at once. If he does not do so within reasonable time, he loses his inviolability and is liable to be fired on or to be made prisoner of war. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 10.10–10.11.