Brexit, a portmanteau of “British” and “exit”, is the impending withdrawal of the United Kingdom (UK) from the European Union (EU). It follows the referendum of 23 June 2016 when 51.9 per cent of those who voted supported withdrawal. Withdrawal has been advocated by Eurosceptics, both left-wing and right-wing, while Pro-Europeanists (or European Unionists), who also span the political spectrum, have advocated continued membership. The UK joined the European Communities (EC) in 1973 under the Conservative government of Edward Heath, with continued membership endorsed by a referendum in 1975. In the 1970s and 1980s, withdrawal from the European Communities was advocated mainly by the political left, with the Labour Party's 1983 election manifesto advocating complete withdrawal. In the late 1980s, opposition to the development of the EC into an increasingly political union grew on the right, with Margaret Thatcher – despite being a key proponent of the European single market – becoming increasingly ambivalent towards Europe. From the 1990s, opposition to further European integration came mainly from the right, and divisions within the Conservative Party led to rebellion over the Maastricht Treaty in 1992.