Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 64. Conclusion of an Agreement to Suspend Combat with the Intention of Attacking by Surprise the Adversary Relying on It
The UK Military Manual (1958) states: “Good faith, as expressed in the observance of promises, is essential in war, for without it hostilities could not be terminated with any degree of safety short of the total destruction of one of the contending parties.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 308.
The manual also states: “In general, it is contrary to modern practice to attempt to obtain advantage of the enemy by deliberate lying, for instance, by declaring that an armistice has been agreed upon when in fact that is not the case.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 314.
The manual further specifies: “To demand a suspension of arms and then to break it by surprise, or to violate a safe conduct or any other agreement, in order to obtain an advantage, is an act of perfidy and as such forbidden”. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 316.
The manual also provides: “It would be perfidy to denounce an armistice for a motive or under a pretext more or less specious and to surprise the enemy without giving him time to put himself on his guard.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 459.
Lastly, the manual states: “In addition to the ‘grave breaches’ of the 1949 [Geneva] Conventions, … the following are examples of punishable violations of the laws of war, or war crimes: … violation of surrender terms.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 626(n).
The UK LOAC Manual (2004) states:
It would be perfidy to denounce an armistice for a specious motive or pretext and to surprise the adverse party without giving him time to put himself on his guard. On the other hand, the existence of an armistice is no reason for relaxing either vigilance or the readiness of troops for action, or for revealing positions to the enemy that he could not detect during combat. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 10.28.