With regard to the applicability of customary international law in the UK, it is an established principle that customary rules are admissible and enforceable in the English courts provided that they do not contravene an Act of Parliament or judicial authority.
The legal position dates back to the eighteenth century, when the Court of Kings Bench described the law of nations as an essential part of English law. Before the issue of enforceability can be considered the courts have to establish the existence of the particular customary rule, and the scope of its effect in municipal law.
The latter requirement clearly evidences the subsequent invasion of the dualistic approach into a sphere which generally rests upon monistic views. If a customary principle is inconsistent with a statutory or common law rule, it will not be applied. However, as Lord Denning MR pointed out, a customary international principle can override a previous judicial decision when said decision is based on an old rule of international law. He further stressed that the doctrine of incorporation is an effective principle of English law, on the basis that the courts have previously recognised changes in the rules of international law without the need of an enabling Act of Parliament.
In order for a customary international principle to be invoked as part of English law there must be evidence demonstrating the assent to incorporate it, or the principle in question must be jus cogens. It is suggested that, since 1876, the doctrine of incorporation applies only where customary law has become part of English law either by an Act of Parliament, judicial authority or by established usage. This is evidenced by examining the judgment of Cockburn CJ. After considering the provisions of English law, he seeks to ascertain whether there is a rule in international law that states otherwise. Assuming the existence of said rule, it is then incumbent on the court to seek evidence of British assent to the relevant rule of international law. However, academic opinion suggests that Cockburn’s judgment is not a diversion from the doctrine of incorporation. It merely stresses upon finding existing evidence of assent of the relevant international provision when the issue affects the liberty of persons.
Generally, the courts pay due regard to the existence of international doctrines. However, in order for a customary international principle to be invoked as part of English law there must be evidence demonstrating the assent to incorporate it, or the principle in question must be jus cogens. This position was affirmed by Atkin LJ in two significant decisions in which he stressed the importance of seeking existing evidence of implementation. In his view, there is no external power that imposes the rules of international law upon English law. As such, an international provision has no validity unless it has been incorporated into domestic law via one of the aforementioned tools.
However, this does not connote that the courts will not consider a novel international provision that has never been brought to the attention of the legislature or judiciary. In that regard, according to Atkin, the court should ascertain the nature of the rule and, having established the rule, they will incorporate it into domestic law provided it does not contradict established case law or enacted statute.
By examining Atkin’s ratio, one can conclude that the courts follow a hybrid mono-dualistic approach. On one hand, they recognise the existence of international customary laws and are prepared to incorporate them in municipal law. On the other hand, they treat the doctrine of incorporation as subordinate to the doctrines of parliamentary sovereignty and stare decisis.
In contrast, with regard to the position of international treaties in English law, the approach is unambiguously dualistic. Treaties are only implemented in English law if Parliament has passed an Act to that effect. In that regard, the doctrine of parliamentary sovereignty is clearly overwhelming. In cases where a treaty provision is inconsistent with an Act of Parliament, notwithstanding the fact that the treaty may be the preceding statutory instrument, the court shall follow the provisions of the English legislation.
Treaties are only implemented in English law if Parliament has passed an Act to that effect. This is treated as a principle of constitutional law and not a rule of construction. This distinction is an important one. In situations where Parliament passes legislation seeking to implement an international treaty, the courts will not apply the concept of parliamentary sovereignty as a rule of construction, since this would distort the primary purpose of enacting the legislation. Instead, the courts apply as a rule of construction the presumption that Parliament intended to fulfil its international obligations. This is exhibited in situations where a provision in the domestic legislation is ambiguous.
However, if the provision is clear, the courts will not refer to the text in the international instrument. This is on the basis that parliamentary sovereignty extends to breaking treaties, in which case the party in detriment shall seek recourse in the legislature and not the judiciary. In order to resort to international legislation, the terms of the domestic legislation must be subject to more than one interpretation, and there must be peremptory extrinsic evidence of an intention to enact the international legislation in question.
However, there is no need for the English statute to expressly grant permission to refer to the international legislation, as this would create the perverse presumption that Parliament intended to break an international convention. It is also important to note that the party relying on the international provision has the burden of adducing evidence of the intention to enact the rule. Even if there is no ambiguity in the domestic provision, the court may refer to the text of the international legislation in order to provide context. For example, when there are discrepancies between the English text in the domestic statute and the foreign text in the convention, the courts would apply the latter since Parliament had intended to give effect to an exact translation of the official language recognised by the convention.
Although in relation to the position of treaties and conventions in English law the courts manifestly follow a dualistic approach, this does not preclude them from referring to unincorporated conventions to resolve ambiguities in domestic law when the issue affects human rights. It follows that the court can refer to an international statutory instrument, such as the European Convention on Human Rights, in order to resolve an ambiguity in domestic primary or enacting legislation, examine the principles upon which the courts should act in exercising discretion, or resolve an uncertainty in common law.
The court can also refer to conventions in order to determine the intention of Parliament behind enacting domestic legislation, when said intention is particularly relevant to the outcome of the case. Even if the common law is certain, the courts would still consider whether there has been a breach of the convention. However, the courts cannot use a provision in a convention for the purposes of deciding the authority of the executive to take a particular action, when said authority is clearly expressed by the domestic legislation. As such, the courts do not apply a presumption that Parliament had the intention to subject the authority to the limitation that it should be exercised consistently with the terms of a convention.
However, there is a different position with regard to European Union legislation, due to the effect of the European Communities Act 1972. As a consequence, the dominant and overwhelming effect of the doctrine of parliamentary sovereignty has been effectively limited. This is demonstrated by the decision in Factortame (No. 2) where the House of Lords, following a reference to the European Court of Justice, stated that the courts have jurisdiction to override national legislation when necessary to grant interim relief in protection of rights conferred under EU law. This rule lies on the basis that Parliament, when passing the 1972 Act, voluntarily limited their sovereignty for the economic benefit of joining the union. The courts cannot use a provision in a convention for the purposes of deciding the authority of the executive to take a particular action, when said authority is clearly expressed by the domestic legislation.
Thus, it is incumbent on a domestic court to override any municipal law which does not conform to a directly enforceable rule of EU law.
 Commercial and Estates Co. of Egypt v Board of Trade  1 KB 271;Chung Chi Cheung v The King  AC 160 at 167-168
 I. Brownlie; Principles of Public International Law (7th edn; Oxford University Press, Oxford, 2008) p.45 – this rule does not apply to treaties relating to the conduct of war or treaties of cession.
 The doctrine is explained by AV Dicey in Introduction to the Study of the Law of the Constitution (Macmillan and Co, London, 1902)
 IRC v Collco Dealings Ltd  AC 1
 Salomon v Commissioners of Customs and Excise  2 QB 116,CA per Lord Denning MR at 141
 Ellerman Lines v Murray  AC 126 per Lord Tomlin at 147
 Salomon v Commissioners of Customs and Excise (supra) per Diplock LJ at 142
 Salomon v Commissioners and Excise (supra) per Diplock LJ at 143,144
 Per Lord Oliver in International Tin Council (J.H. Rayner (Mincing Lane) Ltd v DTI  2 AC 418
Wilson, Smithett and Cope Ltd v Terruzzi  1 All ER 817 – the court referred to Article VIII, s 2(b) of the Bretton Woods Fund Agreement for the establishment of the International Monetary Fund incorporated by The Bretton Woods Agreement Order in Council, 1946 ((SR & O) 1946 No 36 on the issue whether the words in S.2(b) ‘exchange contracts’ connoted contracts of exchange of foreign currency only.
 Corocraft Ltd v Pan American Airways Inc.  1 QB 616
 Derbyshire County Council v Times Newspapers Ltd  1 All ER 1011, HL per Lord Keith of Kinkel at 1020 approving the judgment of Balcombe LJ in the Court of Appeal  3 All ER 65 at 76, 77
 R v Secretary of State for the Home Department, ex parte Brind  1 A.C. 696, HL per Lord Bridge of Harwich at 747-748 and Lord Roskill at 749 – 750
 Attorney General v Guardian Newspaper ltd  1 WLR 100, CA, per Lord Templeman at 103
 AG v Guardian Newspaper Ltd (No2)  1AC 109 – referring to Article 10 ECHR in considering the extent of the duty of confidence
 Waddington v Miah  2 All ER 377, HL per Lord Reid at 379
 R v Chief Metropolitan Stipendiary Magistrate, ex parte Choudhury  1 QB 429, CA per Watkins LJ at 499
 R v Secretary of State for the Home Department, ex parte Brind  1 All ER 469, CA per Lord Donaldson of Lymington MR at 477-478 and Ralph Gibson LJ at 484-486
 Per s 2(4) all UK legislation shall have effect subject to directly applicable EU laws
 M. Dixon and R. McCorquodale; Cases & Materials on International law (4th edn; Oxford University Press, Oxford, 2003) p.122
 Factortame Ltd v Secretary of State for Transport (No.2)  1 All ER 70,HL per Lord Bridge of Harwich at 106-108
 Note that the term ‘directly enforceable’ by definition excludes Directives since they require implementation in domestic law