The Westminster system, or Westminster model, is a type of parliamentary government that incorporates a series of procedures for operating a legislature, first developed in England. Key aspects of the system include an executive branch made up of members of the legislature, and that is responsible to the legislature; the presence of parliamentary opposition parties; and a ceremonial head of state who is separate from the head of government. The term derives from the Palace of Westminster, which has been the seat of the Westminster Parliament in England and later the United Kingdom since the 13th century. The Westminster system is often contrasted with the presidential system that originated in the United States, or with the semi-presidential system, based on the government of France.

The Parliament of the United Kingdom of Great Britain and Northern Ireland is the supreme legislative body of the United Kingdom, and may also legislate for the Crown Dependencies and the British Overseas Territories. It meets at the Palace of Westminster in London. Parliament possesses legislative supremacy and thereby holds ultimate power over all other political bodies in the United Kingdom and the Overseas Territories. While Parliament is bicameral, it has three parts: the sovereign, the House of Lords, and the House of Commons. The three parts acting together to legislate may be described as the King-in-Parliament. The Crown normally acts on the advice of the prime minister, and the powers of the House of Lords are limited to only delaying legislation; thus power is de facto vested in the House of Commons.