The supreme legislature in Great Britain and Northern Ireland, comprising the sovereign, as head of state, and the two chambers, the House of Commons and the House of Lords. Together, these chambers make up the Houses of Parliament, which occupy the Palace of Westminster.
Beginning in the 13th century as simply a formal meeting of the king and certain of his officials and principal lords, Parliament became partly representative, as in Simon de Montfort’s Parliament (1265), which contained commoners (knights of the shire and burgesses of the boroughs) who were elected in their locality, and in Edward I’s Model Parliament (1295).
Until the 16th century, both chambers grew in importance vis-à-vis the crown, as it came to be accepted that their approval was needed for grants of taxation; Henry VIII effected the English Reformation through the long-lived Reformation Parliament (1529–36). Kings such as Charles I tried to manage without summoning a parliament (1629–40), but by the 17th century the Commons had made themselves indispensable. Charles I had to call Parliament in 1640 in order to raise money, and Parliament, led by John Pym, led the opposition to him. The Parliamentary side won the English Civil War, and at the end of the Commonwealth period it was the members of the House of Commons who negotiated the Restoration of Charles II (1660) and the accession of William III and Mary (1688). The legislation enacted in the Glorious Revolution of 1688–89 and the Act of Settlement (1701) settled the relationship of crown, Lords, and Commons definitively and made clear the ultimate supremacy of the Commons.
Present-day workings of Parliament may be summarized as follows. The Prime Minister and the cabinet (a selected group of ministers from either House) are responsible for formulating the policy of the government. Acts of Parliament in draft form, known as Bills, each of which have to be ‘read’ (debated) three times in each House, are referred in the House of Commons (and occasionally in the House of Lords) for detailed consideration to parliamentary standing or select committees. The sovereign’s powers of government are dependent on the advice of ministers, who in turn are responsible to Parliament. The monarch’s prerogatives, exercised through the cabinet or the Privy Council, include the summoning and dissolution of Parliament.
In the 20th century the previous omnicompetence of Parliament was reduced. In Northern Ireland the subordinate Stormont Parliament was established in 1921 to legislate on internal matters; it was suspended in 1972, with a new Northern Ireland Assembly being established in 1999. In Scotland and Wales devolved assemblies were likewise established in 1999. None of these assemblies challenged the ultimate sovereignty of Parliament; but the Treaty of Rome, which the UK accepted in 1972 prior to joining the European Community (now the European Union), provided for a gradual development of Community institutions that took precedence over national sovereignty. Similarly, the European Convention on Human Rights, which came into force in 1953, made the rulings of its court regarding violations of the convention binding on member states. The view that these bodies posed an unacceptable challenge to UK parliamentary sovereignty became a matter of increased controversy in the early 21st century.
The British parliamentary system was adopted by many European countries and by most member countries of the Commonwealth of Nations when they gained dominion status or independence.