Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 53. Starvation as a Method of Warfare
Section C. Blockades and embargoes that cause starvation
The UK LOAC Manual (2004) states:
The declaration or establishment of a blockade is prohibited if:
a. it is intended to starve the civilian population or deny it objects essential for its survival; or
b. the damage to the civilian population is, or may be expected to be, excessive in relation to the concrete and direct military advantage anticipated from the blockade. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 13.74.
With regard to internal armed conflict, the manual states:
15.19. Starvation of civilians as a method of warfare is prohibited.
15.19.1. The right to life is a non-derogable human right. Violence to the life and person of civilians is prohibited, whatever method is adopted to achieve it. It follows that the destruction of crops, foodstuffs and water sources, to such an extent that starvation is likely to follow, is also prohibited. The same applies to sieges, blockades, embargoes, or the blocking of relief supplies with the intention of causing starvation. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 15.19–15.19.1.