The United Kingdom European Union membership referendum, commonly referred to as the EU referendum or the Brexit referendum, took place on 23 June 2016 in the United Kingdom (UK) and Gibraltar to ask the electorate whether the country should remain a member of, or leave, the European Union (EU). It was organised and facilitated through the European Union Referendum Act 2015 and the Political Parties, Elections and Referendums Act 2000. The referendum resulted in 51.9% of the votes cast being in favour of leaving the EU. Although the referendum was legally non-binding, the government of the time promised to implement the result.Membership of the EU had long been a topic of debate in the United Kingdom. The country joined the European Communities (EC), principally the European Economic Community (EEC) or Common Market, the forerunner to the European Union, in 1973, along with the European Coal and Steel Community (ECSC), and the European Atomic Energy Community (EAEC or Euratom). A referendum on continued membership of the Communities was held in 1975, with 67.2% of the votes cast in favour of Britain remaining a member.In May 2015, following a Conservative Party manifesto pledge, the legal basis for the EU referendum was established through the European Union Referendum Act 2015. Britain Stronger in Europe became the official group campaigning for the UK to remain in the EU, and was endorsed by the Prime Minister David Cameron and Chancellor George Osborne. Vote Leave was the official group campaigning for the UK to leave the EU, and was fronted by Conservative MPs Boris Johnson and Michael Gove along with Labour MP Gisela Stuart. Other campaign groups, political parties, businesses, trade unions, newspapers and prominent individuals were also involved, with both sides having supporters from across the political spectrum. Parties in favour of 'remain' included Labour, the Liberal Democrats, the Scottish National Party (SNP), Plaid Cymru and the Green Party; while the UK Independence Party (UKIP) campaigned in favour of leaving the European Union; and the Conservative Party remained neutral. In spite of the official positions of the Conservative Party and Labour, both parties allowed their MPs to publicly campaign for either side of the issue.Immediately after the result, financial markets reacted negatively worldwide, and Cameron announced that he would resign as Prime Minister and Leader of the Conservative Party, having campaigned unsuccessfully to remain in the European Union. It was the first time that a national referendum result had gone against the preferred option of the UK Government. Cameron was succeeded by Theresa May on 13 July 2016. Labour leader Jeremy Corbyn also faced a leadership challenge as a result of the EU referendum. Parliament gave legal effect to the referendum by passing the European Union (Notification of Withdrawal) Act 2017 and on 29 March 2017, the United Kingdom gave formal notice of intent to withdraw from the EU.

The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a sovereign country in north-western Europe, off the north-­western coast of the European mainland. The United Kingdom includes the island of Great Britain, the north-­eastern part of the island of Ireland, and many smaller islands within the British Isles. Northern Ireland shares a land border with the Republic of Ireland. Otherwise, the United Kingdom is surrounded by the Atlantic Ocean, with the North Sea to the east, the English Channel to the south and the Celtic Sea to the south-west, giving it the 12th-longest coastline in the world. The Irish Sea separates Great Britain and Ireland. The total area of the United Kingdom is 93,628 square miles (242,500 km2), with an estimated population in 2020 of over 67 million.The United Kingdom is a unitary parliamentary democracy and constitutional monarchy. The monarch, Queen Elizabeth II, has reigned since 1952. The capital and largest city is London, a global city and financial centre with a metropolitan area population of 14 million. Other major cities include Birmingham, Manchester, Glasgow, Liverpool and Leeds.

The United Kingdom consists of four countries: England, Scotland, Wales and Northern Ireland. Other than England, the constituent countries have their own devolved governments, each with varying powers.The United Kingdom has evolved from a series of annexations, unions and separations of constituent countries over several hundred years. The Treaty of Union between the Kingdom of England (which included Wales, annexed in 1542) and the Kingdom of Scotland in 1707 formed the Kingdom of Great Britain. Its union in 1801 with the Kingdom of Ireland created the United Kingdom of Great Britain and Ireland. Most of Ireland seceded from the UK in 1922, leaving the present United Kingdom of Great Britain and Northern Ireland, which formally adopted that name in 1927.The nearby Isle of Man, Guernsey and Jersey are not part of the UK, being Crown Dependencies with the British Government responsible for defence and international representation. There are also 14 British Overseas Territories, the last remnants of the British Empire which, at its height in the 1920s, encompassed almost a quarter of the world's landmass and a third of the world's population, and was the largest empire in history. British influence can be observed in the language, culture and the legal and political systems of many of its former colonies.The United Kingdom has the world's fifth-largest economy by nominal gross domestic product (GDP), and the tenth-largest by purchasing power parity (PPP). It has a high-income economy and a very high human development index rating, ranking 13th in the world. The UK became the world's first industrialised country and was the world's foremost power during the 19th and early 20th centuries. Today the UK remains one of the world's great powers, with considerable economic, cultural, military, scientific, technological and political influence internationally. It is a recognised nuclear state and is ranked fourth globally in military expenditure. It has been a permanent member of the United Nations Security Council since its first session in 1946.

The United Kingdom is a member of the Commonwealth of Nations, the Council of Europe, the G7, the Group of Ten, the G20, the United Nations, NATO, AUKUS, the Organisation for Economic Co-operation and Development (OECD), Interpol, and the World Trade Organization (WTO). It was a member state of the European Communities (EC) and its successor, the European Union (EU), from its accession in 1973 until its withdrawal in 2020 following a referendum held in 2016.