Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 63. Use of Flags or Military Emblems, Insignia or Uniforms of Neutral or Other States Not Party to the Conflict
The UK LOAC Manual (2004) states:
It is prohibited … to make use in an armed conflict of the flags or military emblems, insignia or uniforms of neutral or other States not Parties to the conflict … The prohibition of use of such items of states not party to the conflict is absolute. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 5.11.
In its chapter on maritime warfare, the manual states: “Warships and auxiliary vessels, however, are prohibited from launching an attack whilst flying a false flag, and at all times from actively simulating the status of those vessels exempt from attack.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 13.82.