Tony Blair's term as the prime minister of the United Kingdom began on 2 May 1997 when he accepted an invitation of Queen Elizabeth II to form a government, succeeding John Major of the Conservative Party, and ended on 27 June 2007 upon his resignation. While serving as prime minister, Blair also served as the first lord of the treasury, minister for the civil service and leader of the Labour Party. He and Gordon Brown both extensively used the New Labour branding while in office, which was presented as the brand of a newly reformed party that had altered Clause IV and endorsed market economics. Blair is the second longest serving prime minister in modern history after Margaret Thatcher, and is the longest serving Labour politician to have held the office.

Sir Anthony Charles Lynton Blair is a British politician who served as Prime Minister of the United Kingdom from 1997 to 2007 and Leader of the Labour Party from 1994 to 2007. He served as Leader of the Opposition from 1994 to 1997 and held various shadow cabinet posts from 1987 to 1994. Blair was Member of Parliament (MP) for Sedgefield from 1983 to 2007. He is the second-longest-serving prime minister in post-war British history and the longest-serving Labour politician to have held the office.