The Politics of the United Kingdom of Great Britain and Northern Ireland takes place in the framework of a constitutional monarchy in which the Monarch is head of state and the Prime Minister of the United Kingdom is the head of government. Executive power is exercised by the UK government and the devolved government of Scotland and Executives of Wales and Northern Ireland. Legislative power is vested in both the government and the two chambers of Parliament, the House of Commons and the House of Lords, as well as in the Scottish parliament and Welsh and Northern Irish assemblies. The judiciary is independent of the executive and the legislature, though several senior judges are still members of the House of Lords, which is currently the highest court of the UK for civil cases and the highest court of England and Wales for criminal cases. Starting in 2009 however, the judicial role of the House of Lords will be scrapped under the Constitutional Reform Act 2005.

The UK is a pluriform multi-party system and since the 1920s, the two largest political parties have been the Labour Party and the Conservative Party. Though coalition and minority governments have been an occasional feature of Parliamentary politics, the first-past-the-post electoral system used for general elections tends to maintain the dominance of these two parties, though each has in the past century relied upon a third party to deliver a working majority in Parliament.

The Liberal Democrats, a party formed by the merger of the former Liberal Party and Social Democratic Party in 1988, is the third largest party in the British parliament. It seeks a reform of the electoral system to address the dominance of the two-party system.

Growing support for 'Nationalist' parties in Scotland and Wales in the 1970s led to proposals for devolution to both countries but devolution only became a reality in the 1990s. Today, Scotland, Wales and Northern Ireland each possess a legislature and government alongside that of the United Kingdom, responsible for devolved matters. However, this increased autonomy and devolution of executive and legislative powers has not contributed to a reduction in support for full independence. Indeed, the principal pro-independence party, the Scottish National Party, won 20 extra MSPs at the 2007 Scottish parliament elections and now forms the Scottish Government.

The constitution is uncodified, being made up of constitutional conventions, statutes and other elements.

This system of government, known as the Westminster system, has been adopted by other countries as well, such as Canada, India, Australia, New Zealand, Singapore, Malaysia and Jamaica, countries that made up part of the British Empire.