On 23 June 2016, British voters chose to leave the European Union in a historic referendum. With a turnout of 72.2 percent, 51.9 percent voted Leave while 48.1 percent voted Remain. The result stunned many observers and triggered immediate economic uncertainty: the pound fell to its lowest level in 31 years, and the FTSE 100 plunged over 3 percent in opening trades. Prime Minister David Cameron, who had campaigned to remain, announced his resignation within hours.

The referendum had been authorized under the European Union Referendum Act 2015, itself a major campaign promise from the Conservative Party. The Leave campaign emphasized immigration control, sovereignty, and reclaiming decision-making power from Brussels. The Remain campaign warned of economic damage and diminished global influence. The vote split the nation sharply along regional, generational, and educational lines: London, Scotland, and younger voters leaned Remain, while older voters and many English regions backed Leave.

The result set in motion "Brexit," a process far more complex than the simple referendum question. Britain formally triggered exit negotiations in March 2017 and did not complete its withdrawal until 31 January 2020. The referendum reshaped British politics, toppled Cameron's government, and created years of parliamentary gridlock over the terms of departure. For the EU, it marked the first time any member state had voted to leave. The referendum demonstrated the power of populist movements against established institutions and presaged similar anti-establishment votes in other democracies that year.