The Prime Minister of the United Kingdom of Great Britain and Northern Ireland is the head of Her Majesty's Government in the United Kingdom. The Prime Minister and Cabinet (consisting of all the most senior ministers, who are government department heads) are collectively accountable for their policies and actions to the Monarch, to Parliament, to their political party and ultimately to the electorate. The current Prime Minister^[update], David Cameron, leader of the Conservative Party, was appointed by the Queen on 11 May 2010.

The office is not established by any constitution or law but exists only as per long-established convention, which stipulates that the monarch must appoint as prime minister the person most likely to command the confidence of the House of Commons; this individual is typically the leader of the political party or coalition of parties that holds the largest number of seats in that chamber.

The position of Prime Minister was not created; it evolved slowly and erratically over three hundred years due to numerous acts of Parliament, political developments, and accidents of history. The office is therefore best understood from a historical perspective. The origins of the position are found in constitutional changes that occurred during the Revolutionary Settlement (1688–1720) and the resulting shift of political power from the Sovereign to Parliament. Although the Sovereign was not stripped of the ancient prerogative powers and legally remained the head of government, politically it gradually became necessary for him or her to govern through a Prime Minister who could command a majority in Parliament.

A prime minister is the most senior minister of cabinet in the executive branch of government in a parliamentary system. In many systems, the prime minister selects and may dismiss other members of the cabinet, and allocates posts to members within the government. In most systems, the prime minister is the presiding member and chairman of the cabinet. In a minority of systems, notably in semi-presidential systems of government, a prime minister is the official who is appointed to manage the civil service and execute the directives of the head of state.

In parliamentary systems fashioned after the Westminster system, the prime minister is the presiding and actual head of the government and head of the executive branch. In such systems, the head of state or the head of state's official representative (i.e. the monarch, president, or governor-general) usually holds a largely ceremonial position, although often with reserve powers.

The prime minister is often, but not always, a member of parliament and is expected with other ministers to ensure the passage of bills through the legislature. In some monarchies the monarch may also exercise executive powers (known as the royal prerogative) which are constitutionally vested in the crown and may be exercised without the approval of parliament.

David William Donald Cameron (pronunciation: /ˈkæmərən/; born 9 October 1966) is the Prime Minister of the United Kingdom, First Lord of the Treasury, Minister for the Civil Service and Leader of the Conservative Party. He represents Witney as its Member of Parliament (MP).

Cameron studied Philosophy, Politics and Economics (PPE) at Oxford, gaining a first class honours degree. He then joined the Conservative Research Department and became Special Adviser to Norman Lamont, and then to Michael Howard. He was Director of Corporate Affairs at Carlton Communications for seven years.

He was defeated in his first candidacy for Parliament at Stafford in 1997, but was elected in 2001 as the Member of Parliament for the Oxfordshire constituency of Witney. He was promoted to the Opposition front bench two years later, and rose rapidly to become head of policy co-ordination during the 2005 general election campaign. With a public image of a youthful, moderate candidate who would appeal to young voters, he won the Conservative leadership election in 2005.