Palace of Westminster in the context of "Burning of Parliament"

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 the 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 Westminster system, or Westminster model, is a type of parliamentary government modelled on that of the Parliament of the United Kingdom. Key aspects of the system include an executive branch made up of members of the legislature which is responsible to the legislature; the presence of parliamentary opposition parties; and a ceremonial head of state who is separate from the head of government. The term derives from the Palace of Westminster, the seat of the British parliament. The Westminster system can be contrasted with the presidential system, which originated in the United States, and with the semi-presidential system based on the government of France.

The Westminster system is used, or was once used, in the national and subnational legislatures of most former colonies of the British Empire upon gaining self-government, beginning with the Province of Canada in 1848. However, many former colonies have since adopted other forms of government.

The Kingdom of Great Britain, officially Great Britain, was a sovereign state in Western Europe from 1707 to the end of 1800. The state was created by the 1706 Treaty of Union and ratified by the Acts of Union 1707, which united the Kingdom of England (including Wales) and the Kingdom of Scotland to form a single kingdom encompassing the whole island of Great Britain and its outlying islands, with the exception of the Isle of Man and the Channel Islands. The unitary state was governed by a single parliament at the Palace of Westminster, but the distinct legal systems—English law and Scots law—remained in use, as did distinct educational systems and religious institutions, namely the Church of England and the Church of Scotland remaining as the national churches of England and Scotland respectively.

The formerly separate kingdoms had been in personal union since the Union of the Crowns in 1603 when James VI of Scotland became King of England and King of Ireland. Since the reign of James, who had been the first to refer to himself as "king of Great Britain", a political union between the two mainland British kingdoms had been repeatedly attempted and aborted by both the Parliament of England and the Parliament of Scotland. Queen Anne (r. 1702–1714) did not produce a clear Protestant heir and endangered the line of succession, with the laws of succession differing in the two kingdoms and threatening a return to the throne of Scotland of the Roman Catholic House of Stuart, exiled in the Glorious Revolution of 1688.

A legislature (UK: /ˈlɛdʒɪslətʃər/, US: /-ˌleɪtʃər/) is a deliberative assembly that holds the legal authority to make law and exercise political oversight within a political entity such as a state, nation, or city. Legislatures are among the principal institutions of state, typically contrasted with the executive and judicial institutions. They may exist at different levels of governance—national, subnational (state, provincial, or regional), local, or supranational—such as the European Parliament.

In most political systems, the laws enacted by legislatures are referred to as primary legislation. Legislatures may also perform oversight, budgetary, and representative functions. Members of a legislature, called legislators, may be elected, indirectly chosen, or appointed, and legislatures may be unicameral, bicameral, or multicameral, depending on their constitutional design.

The Parliament of the United Kingdom of Great Britain and Northern Ireland is the supreme legislative body of the United Kingdom, and may also legislate for the Crown Dependencies and the British Overseas Territories. Since 1999, varying degree of powers have been devolved to the devolved national parliaments of Northern Ireland, Scotland, and Wales. Each devolved parliament has different devolved powers, with Scotland being the most powerful amongst the three devolved parliaments. The central UK Parliament retains the power to legislate in reserved matters, including broadcasting, defence, and currency.

It meets at the Palace of Westminster in London. Parliament possesses legislative supremacy and thereby holds ultimate power over all other political bodies in the United Kingdom and the Overseas Territories. While Parliament is bicameral, it has three parts: the sovereign, the House of Lords, and the House of Commons. The three parts acting together to legislate may be described as the King-in-Parliament. The Crown normally acts on the advice of the prime minister, and the powers of the House of Lords are limited to only delaying legislation.

The House of Commons is the lower house of the Parliament of the United Kingdom. Like the upper house, the House of Lords, it meets in the Palace of Westminster in London, England. The House of Commons is an elected body consisting of 650 members known as members of Parliament (MPs), who are elected to represent constituencies by the first-past-the-post system and hold their seats until Parliament is dissolved.

The House of Commons of England began to evolve in the 13th and 14th centuries. In 1707 it became the House of Commons of Great Britain after the political union with Scotland, and from 1801 it also became the House of Commons for Ireland after the political union of Great Britain and Ireland. In 1922, the body became the House of Commons of the United Kingdom of Great Britain and Northern Ireland after the independence of the Irish Free State. Under the Parliament Acts 1911 and 1949, the Lords' power to reject legislation was reduced to a delaying power. The government is solely responsible to the House of Commons and the prime minister stays in office only as long as they retain the confidence of a majority of the Commons.

The House of Lords is the upper house of the Parliament of the United Kingdom. Like the lower house, the House of Commons, it meets in the Palace of Westminster in London, England. One of the oldest extant institutions in the world, its origins lie in the early 11th century and the emergence of bicameralism in the 13th century.

In contrast to the House of Commons, membership of the Lords is not generally acquired by election. Most members are appointed for life, on either a political or non-political basis. Hereditary membership was limited in 1999 to 92 excepted hereditary peers: 90 elected through internal by-elections, plus the Earl Marshal and the Lord Great Chamberlain as members ex officio, the sole two officers to directly inherit their seats. The House of Lords also includes up to 26 archbishops and bishops of the Church of England, known as Lords Spiritual. Since 2014, membership may be voluntarily relinquished or terminated upon expulsion.

An act of Parliament in the United Kingdom is primary legislation passed by the UK Parliament in Westminster, London.

An act of Parliament can be enforced in all four of the UK constituent countries (England, Scotland, Wales and Northern Ireland). As a result of devolution the majority of acts that are passed by Parliament increasingly only apply either to England and Wales only, or England only. Generally acts only relating to constitutional and reserved matters now apply to the whole of the United Kingdom.