The powers of the prime minister of the United Kingdom come from several sources of the UK constitution, including both statute and constitutional convention, but not one single authoritative document. They have been described as "...problematic to outline definitively."

The Cabinet of the United Kingdom is the senior decision-making body of His Majesty's Government. A committee of the Privy Council, it is chaired by the Prime Minister and its members include Secretaries of State and other senior ministers. Members of the Cabinet are appointed by the Prime Minister and are by convention chosen from members of the two houses of the Parliament of the United Kingdom, the House of Commons and the House of Lords.