List of national legal systems
Common law is currently in practice in Ireland, most of the United Kingdom (England and Wales and Northern Ireland), Australia, New Zealand, Bangladesh, India (excluding Goa), Pakistan, South Africa, Canada (excluding Quebec), Hong Kong, the United States (on a state level excluding Louisiana), and many other places. In addition to these countries, several others have adapted the common law system into a mixed system. For example, Nigeria operates largely on a common law system, but incorporates religious law.
United Kingdom constitutional law
Like other democratic countries, the principles of international law are a basic component of the UK constitution, both as a primary tool of interpretation of domestic law, and through the UK's consistent support and membership of major international organisations. As far back as the Magna Carta 1215, English law recognised the right to free movement of people for international trade. By 1608, Sir Edward Coke wrote confidently that international commercial law, or the "lex mercatoria", is part of the laws of the realm, while the constitutional crises of the 17th century centred upon Parliament halting the King's attempting to tax international trade without its consent. Similarly in the 18th century, Lord Holt CJ viewed international law as a general tool for interpretation of the common law, while Lord Mansfield in particular did more than any other to affirm that the international "lex mercatoria" "is not the law of a particular country but the law of all nations", and "the law of merchants and the law of the land is the same". In 1774, in "Somerset v Stewart", one of the most important cases in legal history, Lord Mansfield held that slavery was lawful "in no country" and therefore in common law. In modern case law it has been consistently accepted that it "is a principle of legal policy that [UK] law should conform to public international law." The House of Lords stressed that "there is a strong presumption in favour of interpreting English law (whether common law or statute) in a way which does not place the United Kingdom in breach of an international obligation." For example, in "Hounga v Allen" the Supreme Court held that a young lady who had been illegally trafficked to the UK had a right to bring a race discrimination claim against her employers, even though she had herself been in violation of the Immigration Act 1971. In doing so, the court unanimously drew upon international treaties signed by the UK, known as the Palermo Protocols, as well as the European Convention on Human Rights, in interpreting the scope of the common law doctrine of illegality, and held it was no bar for the claimant to assert her legal rights. It has been further debated whether the UK should adopt a theory of that sees international law as part of UK without any further act (a "monist" theory), or whether it should still be required for international law principles to be translated into domestic law (a "dualist" theory). The current position in European Union law is that while international law binds the EU, it cannot undermine fundamental principles of constitutional law or human rights. 
Since the World Wars brought an end to the British Empire and physically destroyed large parts of the country, the UK has consistently supported organisations formed under international law. From the Versailles Treaty 1919, the UK was a founding member of the International Labour Organization, which sets universal standards for people's rights at work. After the failure of the League of Nations and following World War Two, the UK became a founding member of the United Nations, recognised by Parliament through the United Nations Act 1946, enabling any resolution of the Security Council except the use of force to be implemented by an Order in Council. Under the Universal Declaration of Human Rights 1948, the continued colonial occupation, and suppression of democracy and human rights in the British Empire lost any remaining legitimacy under international law, and combined with independence movements this led to its rapid dissolution. Two fundamental treaties, the International Covenant on Civil and Political Rights, and the International Covenant on Economic, Social and Cultural Rights in 1966 saw the UK ratify most rights from the Universal Declaration. Following the Ponsonby Rule from 1924, the Constitutional Reform and Governance Act 2010 section 20 stipulates that a treaty is ratified once it is laid before Parliament for 21 days and no adverse resolution is passed against it. Regionally, the UK participated in drafting the European Convention on Human Rights 1950 which sought to guarantee basic standards of democracy and human rights to preserve peace in post-war Europe. At the same time, following long-held visions for European integration with the UK "at the centre", democratic European countries sought to integrate their economies both to make war impossible, and to advance social progress. In 1972, the UK joined the European Community (renamed the European Union in 1992) and committed to implement EU law in which it participated, in the European Communities Act 1972. In 1995, the UK also became a founding member of the World Trade Organization. To ensure that the European Convention was directly applied by the courts, the Human Rights Act 1998 was passed. It also passed the International Criminal Court Act 2001 to enable prosecution of war criminals, and subjected itself to the jurisdiction of the court. In 2016, however, the UK voted in a referendum on whether to leave the European Union, resulting on a 72.21% turnout in a margin of 48.11% favouring "remain", 51.89% favouring "leave" on unspecified terms (27% of the total UK population). However, large majorities in both Scotland and Northern favoured remaining in the EU, and it was revealed that significant criminal conduct took place in the vote. This led to considerable uncertainty about the UK's future role in the international community.