The position of Prime Minister, the UK's head of government, belongs to the Member of Parliament who can obtain the confidence of a majority in the House of Commons, usually the current leader of the largest political party in that chamber. The Prime Minister and Cabinet are formally appointed by the Monarch to form Her Majesty's Government, though the Prime Minister chooses the Cabinet, and by convention HM The Queen respects the Prime Minister's choices.
The Cabinet is traditionally drawn from members of the Prime Minister's party in both legislative houses, and mostly from the House of Commons, to which they are responsible. Executive power is exercised by the Prime Minister and Cabinet, all of whom are sworn into the Privy Council of the United Kingdom, and become Ministers of the Crown. The Rt. Hon. David Cameron, leader of the Conservative Party, has been Prime Minister, First Lord of the Treasury and Minister for the Civil Service since 11 May 2010. For elections to the House of Commons, the UK is currently divided into 650 constituencies with each electing a single Member of Parliament by simple plurality. General elections are called by the Monarch when the Prime Minister so advises. The Parliament Acts 1911 and 1949 require that a new election must be called within five years of the previous general election.
The UK's three major political parties are the Conservative Party, the Labour Party, and the Liberal Democrats, who won between them 622 out of 650 seats available in the House of Commons: 621 seats at the 2010 general election and 1 more at the delayed by-election in Thirsk and Malton. Most of the remaining seats were won by minor parties that only contest elections in one part of the UK such as the Scottish National Party (Scotland only), Plaid Cymru (Wales only), and the Democratic Unionist Party, Social Democratic and Labour Party, Ulster Unionist Party, and Sinn Féin (Northern Ireland only, though Sinn Féin also contests elections in Ireland). In accordance with party policy, no elected Sinn Féin Member of Parliament has ever attended the House of Commons to speak in the House on behalf of their constituents as Members of Parliament are required to take an oath of allegiance to the Monarch. However, the current five Sinn Féin MPs have since 2002 made use of the offices and other facilities available at Westminster. For elections to the European Parliament, the UK currently has 72 MEPs, elected in 12 multi-member constituencies.