Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 15. The Principle of Precautions in Attack
Section A. Constant care to spare the civilian population, civilians and civilian objects
The UK LOAC Manual (2004) states: “In the conduct of military operations, constant care shall be taken to spare the civilian population, civilians and civilian objects.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 5.32.
In 1938, during a debate in the House of Commons, the UK Prime Minister listed among rules of international law applicable to warfare on land, at sea and from the air the rule that “reasonable care must be taken in attacking these military objectives so that by carelessness a civilian population in the neighbourhood is not bombed”. 
United Kingdom, House of Commons, Statement by the Prime Minister, Sir Neville Chamberlain, 21 June 1938, Hansard, Vol. 337, cols. 937–938.
The UK Government Strategy on the Protection of Civilians in Armed Conflict (2010) states: “IHL requires parties to a conflict to respect and protect civilians. … They must take constant care to spare civilians and civilian objects from the effects of hostilities.” 
United Kingdom, Foreign and Commonwealth Office, Government Strategy on the Protection of Civilians in Armed Conflict, March 2010, p. 4.
While the core challenges in the protection of civilians identified in the previous reports of the Secretary-General still need our sustained attention, the new report also identifies several protection policy priorities that need to be explored. In particular the following “emerging” issues would benefit from our attention, and the Group of Friends stands ready to act as a platform to advance them. …
… [O]n the issue of lethal autonomous weapons systems (LAWS), the Group is of the view that further discussions are needed and it welcomes the fact that the issue will be examined in Geneva in May 2014, in the framework of the CCW [Convention on Certain Conventional Weapons]. The Group hopes that such discussions will also examine the issue with due consideration to the protection of civilians as part of a comprehensive debate including legal, military operational, technological and ethical perspectives. In time discussion should focus on the relevance of such systems to the protection of civilians, in particular in the context of IHL and with regard to the principles of distinction, precaution and proportionality. 
United Kingdom, Statement by the permanent representative of Switzerland during a UN Security Council open debate on the protection of civilians in armed conflict made on behalf of the Group of Friends on the Protection of Civilians in Armed Conflict, namely Australia, Austria, Belgium, Brazil, Canada, France, Germany, Italy, Japan, Liechtenstein, Norway, Portugal, Switzerland, the United Kingdom and Uruguay, 12 February 2014, p. 2.