2011 United Kingdom Alternative Vote referendum

The United Kingdom Alternative Vote referendum, also known as the UK-wide referendum on the Parliamentary voting system, was held on Thursday 5 May 2011 in the United Kingdom to choose the method of electing MPs at subsequent general elections. It occurred as a provision of the Conservative–Liberal Democrat coalition agreement drawn up in 2010 (after a general election that had resulted in the first hung parliament since February 1974) and also indirectly in the aftermath of the 2009 expenses scandal. It operated under the provisions of the Parliamentary Voting System and Constituencies Act 2011 and was the first national referendum to be held under provisions laid out in the Political Parties, Elections and Referendums Act 2000. Many local elections were also held on this day.

The referendum concerned whether to replace the present "first-past-the-post" system with the "alternative vote" (AV) method and was the first national referendum to be held across the whole of the United Kingdom in the 21st century. The proposal to introduce AV was rejected by 67.9% of voters on a national turnout of 42%. The failure of the referendum was considered a humiliating setback for Liberal Democrat leader Nick Clegg, who had acquiesced to the Conservative offer of a referendum on AV rather than proportional representation (PR) as part of the coalition agreement. The referendum was linked to the ongoing decline of his popularity and that of the Liberal Democrats in general.

This was only the second UK-wide referendum to be held (the first was the EC referendum in 1975) and the first of such to be overseen by the Electoral Commission. It is to date the only UK-wide referendum to be held on an issue not related to the European Communities or the European Union, and is also the first to have been not merely consultative: it committed the government to give effect to its decision.