Claim: Tony Blair is a politician.

The Prime Minister of the United Kingdom is the head of Her Majesty 's Government in the United Kingdom . The Prime Minister -LRB- informal abbreviation : PM -RRB- and Cabinet -LRB- consisting of all the most senior ministers , most of whom are government department heads -RRB- are collectively accountable for their policies and actions to the Monarch , to Parliament , to their political party and ultimately to the electorate . The office is one of the Great Offices of State . The , Theresa May , leader of the Conservative Party , was appointed by the Queen on 13 July 2016 .   The office is not established by any constitution or law but exists only by 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 -LRB- 1688 -- 1720 -RRB- 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 .   By the 1830s the Westminster system of government -LRB- or cabinet government -RRB- had emerged ; the Prime Minister had become primus inter pares or the first among equals in the Cabinet and the head of government in the United Kingdom . The political position of Prime Minister was enhanced by the development of modern political parties , the introduction of mass communication -LRB- inexpensive newspapers , radio , television and the internet -RRB- , and photography . By the start of the 20th century the modern premiership had emerged ; the office had become the pre-eminent position in the constitutional hierarchy vis-à-vis the Sovereign , Parliament and Cabinet .   Prior to 1902 , the Prime Minister sometimes came from the House of Lords , provided that his government could form a majority in the Commons . However as the power of the aristocracy waned during the 19th century the convention developed that the Prime Minister should always sit in the lower house . As leader of the House of Commons , the Prime Minister 's authority was further enhanced by the Parliament Act of 1911 which marginalised the influence of the House of Lords in the law-making process .   The Prime Minister is ex officio also First Lord of the Treasury and Minister for the Civil Service . Certain privileges , such as residency of 10 Downing Street , are accorded to Prime Ministers by virtue of their position as First Lord of the Treasury .