The Commons is an elected body consisting of 650 members known as Members of Parliament (MPs). Members are elected to represent constituencies by first-past-the-post and hold their seats until Parliament is dissolved.
The House of Commons of England evolved in the 13th and 14th centuries. It eventually became the House of Commons of Great Britain after the political union with Scotland in 1707, and assumed the title of "House of Commons of Great Britain and Ireland" after the political union with Ireland at the start of the 19th century. The "United Kingdom" referred to was the United Kingdom of Great Britain and Ireland from 1800, and became the United Kingdom of Great Britain and Northern Ireland after the independence of the Irish Free State in 1922. Accordingly, the House of Commons assumed its current title.
Under the Parliament Act 1911, the Lords' power to reject legislation was reduced to a delaying power. The Government is solely responsible to the House of Commons and the Prime Minister stays in office only as long as he or she retains the support of a majority of the Commons.
Although it does not formally elect the prime minister, the position of the parties in the House of Commons is of overriding importance. By convention, the prime minister is answerable to, and must maintain the support of, the House of Commons. Thus, whenever the office of prime minister falls vacant, the Sovereign appoints the person who has the support of the House, or who is most likely to command the support of the House—normally the leader of the largest party in the Commons, while the leader of the second-largest party becomes the Leader of the Opposition. Since 1963, by convention, the prime minister is always a member of the House of Commons, rather than the House of Lords.
The Commons may indicate its lack of support for the Government by rejecting a motion of confidence or by passing a motion of no confidence. Confidence and no confidence motions are phrased explicitly, for instance: "That this House has no confidence in Her Majesty's Government." Many other motions were considered confidence issues, even though not explicitly phrased as such. In particular, important bills that form a part of the Government's agenda were formerly considered matters of confidence, as is the annual Budget. When a Government has lost the confidence of the House of Commons, the prime minister is obliged either to resign, making way for another MP who can command confidence, or to request the monarch to dissolve Parliament, thereby precipitating a general election.
Parliament normally sits for a maximum term of five years. Subject to that limit, the prime minister could formerly choose the timing of the dissolution of parliament, with the permission of the Monarch. However, since the Fixed-Term Parliaments Act 2011, terms are now a fixed five years, and an early general election can only be brought about either by a two-thirds majority in favour of a motion for a dissolution, or by a vote of no confidence that is not followed within fourteen days by a vote of confidence (which may be for confidence in the same government or a different one). By this second mechanism, the government of the United Kingdom can change without an intervening general election. Only four of the eight last Prime Ministers have attained office as the immediate result of a general election; the others have gained office upon the resignation of a Prime Minister of their own party. The latter four were Jim Callaghan, John Major, Gordon Brown and the current Prime Minister Theresa May; these four inherited the office from Harold Wilson, Margaret Thatcher, Tony Blair and David Cameron respectively. In such circumstances there may not even have been an internal party leadership election, as the new leader may be chosen by acclaim, having no electoral rival (as in the case of both Brown and May).
A prime minister may resign even if he or she is not defeated at the polls (for example, for personal health reasons). In such a case, the premiership goes to whoever can command a majority in the House of Commons; in practice this is usually the new leader of the outgoing prime minister's party. Until 1965, the Conservative Party had no mechanism for electing a new leader; when Anthony Eden resigned as PM in 1957 without recommending a successor, the party was unable to nominate one. It fell to the Queen to appoint Harold Macmillan as the new prime minister, after taking the advice of ministers.