British Prime Minister Margaret Thatcher resigns as leader of the Conservative Party and the prime ministry, and is succeeded as both by John Major.

Margaret Hilda Thatcher, Baroness Thatcher (ne Roberts; 13 October 1925 8 April 2013), was Prime Minister of the United Kingdom from 1979 to 1990 and Leader of the Conservative Party from 1975 to 1990. She was the first female British prime minister and the longest-serving British prime minister of the 20th century. As prime minister, she implemented economic policies that became known as Thatcherism. A Soviet journalist dubbed her the "Iron Lady", a nickname that became associated with her uncompromising politics and leadership style.

Thatcher studied chemistry at Somerville College, Oxford, and worked briefly as a research chemist, before becoming a barrister. She was elected Member of Parliament for Finchley in 1959. Edward Heath appointed her Secretary of State for Education and Science in his 19701974 government. In 1975, she defeated Heath in the Conservative Party leadership election to become Leader of the Opposition, the first woman to lead a major political party in the United Kingdom.

On becoming prime minister after winning the 1979 general election, Thatcher introduced a series of economic policies intended to reverse high inflation and Britain's struggles in the wake of the Winter of Discontent and an oncoming recession. Her political philosophy and economic policies emphasised deregulation (particularly of the financial sector), the privatisation of state-owned companies, and reducing the power and influence of trade unions. Her popularity in her first years in office waned amid recession and rising unemployment. Victory in the 1982 Falklands War and the recovering economy brought a resurgence of support, resulting in her landslide re-election in 1983. She survived an assassination attempt by the Provisional IRA in the 1984 Brighton hotel bombing and achieved a political victory against the National Union of Mineworkers in the 198485 miners' strike.

Thatcher was re-elected for a third term with another landslide in 1987, but her subsequent support for the Community Charge ("poll tax") was widely unpopular, and her increasingly Eurosceptic views on the European Community were not shared by others in her cabinet. She resigned as prime minister and party leader in 1990, after a challenge was launched to her leadership. After retiring from the Commons in 1992, she was given a life peerage as Baroness Thatcher (of Kesteven in the County of Lincolnshire) which entitled her to sit in the House of Lords. In 2013, she died of a stroke at the Ritz Hotel, London, at the age of 87.

A polarising figure in British politics, Thatcher is nonetheless viewed favourably in historical rankings and public opinion of British prime ministers. Her tenure constituted a realignment towards neoliberal policies in Britain, with the complicated legacy attributed to Thatcherism debated into the 21st century.

The prime minister of the United Kingdom is the head of government in the United Kingdom. The prime minister advises the sovereign on the exercise of much of the royal prerogative, chairs the Cabinet and selects its ministers. As modern prime ministers hold office by virtue of their ability to command the confidence of the House of Commons, they sit as a member of Parliament.

The office of prime minister is not established by any statute or constitutional document, but exists only by long-established convention, whereby the reigning monarch appoints 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. It was announced by Prime Minister Boris Johnson on 31 January 2022 that the office of Prime Minister would be formally established.The prime minister is ex officio also First Lord of the Treasury, Minister for the Civil Service and the minister responsible for national security.: p.22  Indeed, 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. In 2019, the office of Minister for the Union was established; Boris Johnson became the first prime minister to hold this title.