Rule 19. Control during the Execution of Attacks
Rule 19. Each party to the conflict must do everything feasible to cancel or suspend an attack if it becomes apparent that the target is not a military objective or that the attack may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated.
Summary
State practice establishes this rule as a norm of customary international law applicable in both international and non-international armed conflicts.
International armed conflicts
The obligation to do everything feasible to cancel or suspend an attack if it becomes apparent that the target is not a military objective or that the attack may be expected to cause excessive incidental damage is set forth in Article 57(2)(b) of Additional Protocol I, to which no relevant reservations have been made.[1]  Upon ratification of Additional Protocol I, the United Kingdom stated that this obligation only applied to “those who have the authority and practical possibility to cancel or suspend the attack”.[2] 
This obligation is included in numerous military manuals.[3]  It is also supported by official statements and reported practice.[4]  This practice includes that of States not, or not at the time, party to Additional Protocol I.[5]  When the ICRC appealed to the parties to the conflict in the Middle East in October 1973, i.e., before the adoption of Additional Protocol I, to do everything feasible to cancel or suspend an attack if it becomes apparent that the target is not a military objective or that the attack may be expected to cause excessive incidental damage, the States concerned (Egypt, Iraq, Israel and Syrian Arab Republic) replied favourably.[6] 
Non-international armed conflicts
While Additional Protocol II does not include an explicit reference to this rule, more recent treaty law applicable in non-international armed conflicts does so, namely the Second Protocol to the Hague Convention for the Protection of Cultural Property.[7]  In addition, this rule is contained in other instruments pertaining also to non-international armed conflicts.[8] 
Military manuals which are applicable in or have been applied in non-international armed conflicts specify the obligation to do everything feasible to cancel or suspend an attack if it becomes apparent that the target is not a military objective or that the attack may be expected to cause excessive incidental damage.[9] 
The jurisprudence of the International Criminal Tribunal for the Former Yugoslavia in the Kupreškić case provides further evidence of the customary nature of this rule in both international and non-international armed conflicts. In its judgment, the Tribunal considered that this rule was customary because it specified and fleshed out general pre-existing norms.[10]  It can be argued indeed that the principle of distinction (see Rules 1 and 7) and the principle of proportionality (see Rule 14), both of which are customary in international and non-international armed conflicts, inherently require respect for this rule. Disregard for this rule would lead to an attack in violation of the principles of distinction and of proportionality and would be illegal on that basis. The Tribunal also relied on the fact that this rule had not been contested by any State.[11]  This study found no official contrary practice either.

[1] Additional Protocol I, Article 57(2)(b) (adopted by 90 votes in favour, none against and 4 abstentions) (cited in Vol. II, Ch. 5, § 367).
[2] United Kingdom, Reservations and declarations made upon ratification of Additional Protocol I (ibid., § 158).
[3] See, e.g., the military manuals of Argentina (ibid., § 373), Australia (ibid., §§ 374–375), Belgium (ibid., § 376), Benin (ibid., § 377), Cameroon (ibid., § 378), Canada (ibid., § 379), Colombia (ibid., § 380), Croatia (ibid., § 381), France (ibid., § 382), Germany (ibid., § 383), Hungary (ibid., § 384), Italy (ibid., § 385), Kenya (ibid., § 386), Madagascar (ibid., § 387), Netherlands (ibid., § 388), New Zealand (ibid., § 389), Spain (ibid., § 390), Sweden (ibid., § 391), Switzerland (ibid., § 392), Togo (ibid., § 393), United Kingdom (ibid., § 394) and United States (ibid., § 395).
[4] See, e.g., the statements of Indonesia (ibid., § 400), Iraq (ibid., § 401), Jordan (ibid., § 403), Netherlands (ibid., § 405), United Kingdom (ibid., § 407) and United States (ibid., §§ 409–411) and the reported practice of Israel (ibid., § 402), Malaysia (ibid., § 404), Syrian Arab Republic (ibid., § 406), United States (ibid., § 408) and Zimbabwe (ibid., § 412).
[5] See, e.g., the military manuals of Kenya (ibid., § 386), United Kingdom (ibid., § 394) and United States (ibid., § 395); the statements of Indonesia (ibid., § 400), Iraq (ibid., § 401), United Kingdom (ibid., § 407) and United States (ibid., §§ 409–411) and the reported practice of Israel (ibid., § 402), Malaysia (ibid., § 404) and United States (ibid., § 408).
[6] See ICRC, The International Committee’s Action in the Middle East (ibid., § 417).
[7] Second Protocol to the Hague Convention for the Protection of Cultural Property, Article 7 (ibid., § 368).
[8] See, e.g., Memorandum of Understanding on the Application of IHL between Croatia and the Socialist Federal Republic of Yugoslavia, § 6 (ibid., § 370); Agreement on the Application of IHL between the Parties to the Conflict in Bosnia and Herzegovina, § 2.5 (ibid., § 371); San Remo Manual, § 46(d) (ibid., § 372).
[9] See, e.g., the military manuals of Australia (ibid., § 374), Benin (ibid., § 377), Colombia (ibid., § 380), Croatia (ibid., § 381), Germany (ibid., § 383), Italy (ibid., § 385), Kenya (ibid., § 386), Madagascar (ibid., § 387) and Togo (ibid., § 393).
[10] ICTY, Kupreškić case, Judgment (ibid., § 416).
[11] ICTY, Kupreškić case, Judgment (ibid., § 416).