Related Rule
Canada
Practice Relating to Rule 19. Control during the Execution of Attacks
Canada’s LOAC Manual (1999) provides:
An attack must be cancelled or suspended if it becomes apparent that the objective is not a legitimate target, or that the attack may be expected to cause collateral civilian damage which would be excessive in relation to the concrete and direct military advantage anticipated. 
Canada, The Law of Armed Conflict at the Operational and Tactical Level, Office of the Judge Advocate General, 1999, p. 4-4, § 28; see also p. 4-3, § 18 (proportionality test) and p. 7-5, § 50 (air to land operations).
Canada’s LOAC Manual (2001) states in its chapter on targeting:
413. Proportionality
2. The proportionality test is as follows: Is the attack expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof (“collateral civilian damage”), which would be excessive in relation to the concrete and direct military advantage anticipated? If the answer is “yes”, the attack must be cancelled or suspended. The proportionality test must be used in the selection of any target.
419. Cancellation or Suspension of an Attack
1. An attack must be cancelled or suspended if it becomes apparent that the objective is not a legitimate target, or that the attack may be expected to cause collateral civilian damage, which would be excessive in relation to the concrete and direct military advantage anticipated. 
Canada, The Law of Armed Conflict at the Operational and Tactical Levels, Office of the Judge Advocate General, 13 August 2001, §§ 413.2 and 419.
In its chapter on air warfare, the manual states:
An attack shall be cancelled or suspended if it becomes apparent that the objective is not a legitimate target or is subject to special protection, or that the attack may be expected to cause collateral civilian damage, which would be excessive in relation to the concrete and direct military advantage anticipated. 
Canada, The Law of Armed Conflict at the Operational and Tactical Levels, Office of the Judge Advocate General, 13 August 2001, § 716.6.