Great Britain is a constitutional monarchy. The constitution exists in no one document but is a centuries-old accumulation of statutes, judicial decisions, usage, and tradition. The hereditary monarch, who must belong to the Church of England according to the Act of Settlement of 1701, is almost entirely limited to exercising ceremonial functions as the head of state.
Sovereignty rests in Parliament, which consists of the House of Commons, the House of Lords, and the crown. Effective power resides in the Commons, whose 650 members are elected from single-member constituencies. The executive—the cabinet of ministers headed by the prime minister, who is the head of government—is usually drawn from the party holding the most seats in the Commons; the monarch usually asks the leader of the majority party to be prime minister. Historically, the hereditary and life peers of the realm, high officials of the Church of England, and the lords of appeal (who exercised judicial functions until a Supreme Court was established in 2009) had the right to sit in the House of Lords, but in 1999 both houses voted to strip most hereditary peers of their right to sit and vote in the chamber. Most legislation originates in the Commons. The House of Lords may take a part in shaping legislation, but it cannot permanently block a bill passed by the Commons, and it has no authority over money bills. The crown need not assent to all legislation, but assent has not been withheld since 1707.
Since 1999 both Scotland and Wales have assumed some regional governmental powers through the institution of a parliament and an assembly, respectively. In addition, Northern Ireland has had home rule through a parliament or assembly at various times since the early 20th cent. The introduction of Scottish and Welsh representative assemblies has raised the question of whether England should have its own parliament, separate from that of the United Kingdom, with powers similar to those of the Scottish body, or of whether Scottish and Welsh members of the British parliament should be barred from voting on matters that affect England only. The issue is controversial, with some fearing that the establishment of a parliament for England would ultimately lead to the dissolution of the United Kingdom.
The two main parties are the Conservative party, descended from the old Tory party, and the Labour party, which was organized in 1906 and is moderately socialist. The Liberal Democrats, formed by the merger of the Liberal party and the Social Democratic party, is a weaker third party. Both Scotland and Wales have nationalist parties whose goal is the independence of those respective regions.