Constitution of the United Kingdom

The constitution of the United Kingdom comprises the written and unwritten arrangements that establish the United Kingdom of Great Britain and Northern Ireland as a political body. Unlike in most countries, no official attempt has been made to codify such arrangements into a single document, thus it is known as an uncodified constitution. This enables the constitution to be easily changed as no provisions are formally entrenched.

The Supreme Court of the United Kingdom and its predecessor, the Appellate Committee of the House of Lords, have recognised and affirmed constitutional principles such as parliamentary sovereignty, the rule of law, democracy, and upholding international law. It also recognises that some Acts of Parliament have special constitutional status. These include Magna Carta, which in 1215 required the King to call a "common counsel" (now called Parliament) to represent the people, to hold courts in a fixed place, to guarantee fair trials, to guarantee free movement of people, to free the church from the state, and to guarantee rights of "common" people to use the land. After the Glorious Revolution, the Bill of Rights 1689 and the Claim of Right Act 1689 cemented Parliament's position as the supreme law-making body, and said that the "election of members of Parliament ought to be free". The Treaty of Union in 1706 and the Acts of Union 1707 united the Kingdoms of England, Wales and Scotland, the Acts of Union 1800 joined Ireland, but the Irish Free State separated after the Anglo-Irish Treaty in 1922, leaving Northern Ireland within the UK. After struggles for universal suffrage, the UK guaranteed every adult citizen over 21 years the equal right to vote in the Representation of the People (Equal Franchise) Act 1928. After World War II, the UK became a founding member of the Council of Europe to uphold human rights, and the United Nations to guarantee international peace and security. The UK was a member of the European Union, joining its predecessor in 1973, but left in 2020. The UK is also a founding member of the International Labour Organization and the World Trade Organization to participate in regulating the global economy.

The leading institutions in the United Kingdom's constitution are Parliament, the judiciary, the executive, and regional and local governments, including the devolved legislatures and executives of Scotland, Wales, and Northern Ireland. Parliament is the supreme law-making body, and represents the people of the United Kingdom. The House of Commons is elected by a democratic vote in the country's 650 constituencies. The House of Lords is mostly appointed by cross-political party groups from the House of Commons, and can delay but not block legislation from the Commons. To make a new Act of Parliament, the highest form of law, both Houses must read, amend, or approve proposed legislation three times and the monarch must give consent. The judiciary interprets the law found in Acts of Parliament and develops the law established by previous cases. The highest court is the twelve-person Supreme Court, as it decides appeals from the Courts of Appeal in England, Wales, and Northern Ireland, or the Court of Session in Scotland. UK courts cannot decide that Acts of Parliament are unconstitutional or invalidate them, but can declare that they are incompatible with the European Convention on Human Rights. They can determine whether the acts of the executive are lawful. The executive is led by the prime minister, who must maintain the confidence of a majority of the members of the House of Commons. The prime minister appoints the cabinet of other ministers, who lead the executive departments, staffed by civil servants, such as the Department of Health and Social Care which runs the National Health Service, or the Department for Education which funds schools and universities.

The monarch in their public capacity, known as the Crown, embodies the state. Laws can only be made by or with the authority of the Crown in Parliament, all judges sit in place of the Crown and all ministers act in the name of the Crown. The monarch is for the most part a ceremonial figurehead and has not refused assent to any new law since the Scottish Militia Bill in 1708. The monarch is bound by constitutional convention.

Most constitutional questions arise in judicial review applications, to decide whether the decisions or acts of public bodies are lawful. Every public body can only act in accordance with the law, laid down in Acts of Parliament and the decisions of the courts. Under the Human Rights Act 1998, courts may review government action to decide whether the government has followed the statutory obligation on all public authorities to comply with the European Convention on Human Rights. Convention rights include everyone's rights to life, liberty against arbitrary arrest or detention, torture, and forced labour or slavery, to a fair trial, to privacy against unlawful surveillance, to freedom of expression, conscience and religion, to respect for private life, to freedom of association including joining trade unions, and to freedom of assembly and protest.

Although the British constitution is not codified, the Supreme Court recognises constitutional principles, and constitutional statutes, which shape the use of political power. There are at least four main constitutional principles recognised by the courts. First, parliamentary sovereignty means that Acts of Parliament are the supreme source of law. Through the English Reformation, the Civil War, the Glorious Revolution of 1688 and the Acts of Union 1707, Parliament became the dominant branch of the state, above the judiciary, executive, monarchy, and church. Parliament can make or unmake any law, a fact that is usually justified by Parliament being democratically elected, and upholding the rule of law, including human rights and international law.

Second, the rule of law has run through the constitution as a fundamental principle from the earliest times as "The king must [be] ... under the law, because the law makes the king" (Henry de Bracton in the 13th century). This principle was recognised in Magna Carta and the Petition of Right 1628. This means the government may only conduct itself according to legal authority, including respect for human rights. Third, at least since 1928, elections in which all capable adults participate have become a fundamental constitutional principle. Originally only wealthy, property-owning men held rights to vote for the House of Commons, while the monarch, occasionally together with a hereditary House of Lords, dominated politics. From 1832 onwards, adult citizens slowly obtained the right to universal suffrage.

Fourth, the British constitution is bound to international law, as Parliament has chosen to increase its practical power in cooperation with other countries in international organisations, such as the International Labour Organization, the United Nations, the European Convention on Human Rights, the World Trade Organization, and the International Criminal Court. However, the UK left membership of the European Union in 2020 after a referendum in 2016.