The House of Commons is the lower house of the Parliament of the United Kingdom. Like the upper house, the House of Lords, it meets in the Palace of Westminster in London, England. The House of Commons is an elected body consisting of 650 members known as members of Parliament (MPs), who are elected to represent constituencies by the first-past-the-post system and hold their seats until Parliament is dissolved.

The House of Commons of England began to evolve in the 13th and 14th centuries. In 1707 it became the House of Commons of Great Britain after the political union with Scotland, and from 1801 it also became the House of Commons for Ireland after the political union of Great Britain and Ireland. In 1922, the body became the House of Commons of the United Kingdom of Great Britain and Northern Ireland after the independence of the Irish Free State. Under the Parliament Acts 1911 and 1949, 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 they retain the confidence of a majority of the Commons.

Although the House of Commons does not formally elect the prime minister, by convention and in practice, the prime minister is answerable to the House, and therefore must maintain its support. In this way, the position of the parties in the House is an overriding importance. Thus, whenever the office of prime minister falls vacant, the monarch 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 house—while the leader of the second-largest party becomes the leader of the Opposition.

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 His Majesty's Government." Many other motions were until recent decades considered confidence issues, even though not explicitly phrased as such: in particular, important bills that were part of the government's agenda. The annual Budget is still considered a matter of confidence. When a government has lost the confidence of the House of Commons, the prime minister is expected either to resign, making way for another MP who can command confidence, or request the monarch to dissolve Parliament, thereby precipitating a general election.

Since the Dissolution and Calling of Parliament Act 2022 Parliament sits for up to five years. Prime ministers can, however, choose to dissolve parliament at earlier times, with the permission of the monarch, and often have. This was a return to the historic system that had been replaced by the Fixed-term Parliaments Act 2011, which fixed the term at five years. As of 9 July 2024, five of the twelve 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.

A prime minister will resign after party defeat at an election, if unable to form a coalition, or obtain a confidence and supply arrangement, and may be forced to resign after a successful motion of no confidence (either from the House as a whole, or from their own parliamentary party). In such cases, the premiership goes to whomever can command a majority in the House, unless there is a hung parliament and a coalition is formed; the new prime minister will by convention be the new leader of the outgoing premier's party. It has become the practice to write the constitutions of major UK political parties to provide a set way to appoint a new party leader.

By convention, ministers are members of either the House of Commons or the House of Lords. A handful have been appointed from outside Parliament, but in most cases they then entered Parliament in a by-election or by receiving a peerage (being made a peer). Since 1902, all but one prime ministers have been members of the Commons at time of appointment; the sole exception was during the long summer recess in 1963: Alec Douglas-Home, then the 14th Earl of Home, disclaimed his peerage (under a new mechanism which remains in force) three days after becoming prime minister. The new session of Parliament was delayed to await the outcome of his by-election, which happened to be already under way due to a recent death. As anticipated, he won that election, which was for the highest-majority seat in Scotland among his party; otherwise he would have been constitutionally obliged to resign.^{[citation needed]}

Since 1990, almost all cabinet ministers, save for three whose offices are an intrinsic part of the House of Lords, have belonged to the Commons.