The Parliamentary Labour Party (PLP) consists of the Labour Party in Parliament: Labour MPs as a collective body.

Commentators on the British Constitution sometimes draw a distinction between the Labour Party (which was created outside Parliament and later achieved office) and the Conservative and Liberal parties (which began as parliamentary factions). The term 'Parliamentary Labour Party' properly refers to the party in parliament, whereas the term Labour Party refers to the entire Labour Party, the parliamentary faction of which is the PLP.

Originally, the Leader of the Labour Party was elected by the PLP. Nowadays, an electoral college is used, consisting of three sections - MPs and MEPs, affiliated organizations (affiliated trade unions and affiliated socialist societies), and Constituency Labour Parties (CLPs). Each section exercises one-third of the vote (meaning that overall, MPs have substantially more say than individual members). Multiple-voting is used, so an MP who belongs to two trade unions, one socialist society, and a CLP receives five votes. The single transferable vote is used to conduct the election.

Labour MPs retain the power to trigger an extraordinary or "special" Labour Party Conference to choose a new leader if they lose confidence in their existing leader.

Labour MPs also elect two of their number to Labour's National Executive Committee.

The PLP holds regular meetings behind closed doors to question the Leader and to discuss its concerns.

A similar body for the Conservative Party is the 1922 Committee.

Chairman of the Parliamentary Labour Party

The Chairman of the PLP chairs meetings of the Parliamentary faction. He or she is elected by Labour MPs at the start of each annual session of Parliament. Usually, by tradition, only elections at the start of each Parliament, following a General Election, are competitive.

From 1921 to 1970, the Chairman of the PLP was also the leader of the party as a whole (before 1921, leadership of the party was arguably split between the Chairman of the PLP, the Secretary of the Party and the Chairman of the Party). When the leaders of the Labour Party joined coalition governments during the First and Second World Wars, an acting chairman was appointed to lead the rump of the party in Opposition. When the Party was in government, a liaison committee was elected to facilitate communications between the cabinet and Labour backbenchers - the chairman of this committee also chaired meetings of the PLP as a whole during these periods. In 1970, the positions of Leader of the Labour Party and Chairman of the PLP were permanently split.

• Keir Hardie (1906-08)
• Arthur Henderson (1908-10)
• George Barnes (1910-11)
• Ramsay Macdonald (1911-14)
• Arthur Henderson (1914-17)
  • John Hodge (1915-16) - in Opposition
  • George Wardle (1916-17) - in Opposition
• William Adamson (1917-21)
• J. R. Clynes (1921-22)
• Ramsay Macdonald (1922-31)
  • Robert Smillie (1924) - Liaison Committee
  • Harry Snell (1929-31) - Liaison Committee
• Arthur Henderson (1931)
• George Lansbury (1931-35)
• Clement Attlee (1935-55)
  • Hastings Lees-Smith (1940-42) - in Opposition
  • Frederick Pethick-Lawrence (1942) - in Opposition
  • Arthur Greenwood (1942-45) - in Opposition
  • Neil Maclean (1945-46) - Liaison Committee
  • Maurice Webb (1946-50) - Liaison Committee
  • Glenvil Hall (1950-51) - Liaison Committee
• Hugh Gaitskell (1955-63)
• Harold Wilson (1963-70)
  • Emanuel Shinwell (1964-67) - Liaison Committee
  • Douglas Houghton (1967-70) - Liaison Committee
• Douglas Houghton (1970-74)
• Ian Mikardo (1974)
• Cledwyn Hughes (1974-79)
• Fred Willey (1979-81)
• Jack Dormand (1981-87)
• Stanley Orme (1987-92)
• Douglas Hoyle (1992-97)
• Clive Soley (1997-01)
• Jean Corston (2001 onwards)