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How do you become British Prime Minister?

Updated Monday, 24th October 2022

Rishi Sunak will be the next Prime Minister of the UK... Ever fancied running the country? Dr Richard Heffernan has some advice on the mechanics of the political system.


Britain, like France, is a unitary state, but is a parliamentary democracy, not a semi-presidential system. British voters thus do not directly elect the government or the head of the government, the Prime Minister. And the head of state, the Monarch, is hereditary, apolitical, and not subject to election or appointment. Voters only elect the part of the legislature, the House of Commons, from which the government emerges. At a general election, electors vote only to elect an MP in 1 of the 650 parliamentary constituencies in which they live. 

They thereby collectively produce the key part of Britain’s legislature, the House of Commons, and it is from the Commons that the executive, the government, then indirectly emerges. Boris Johnson had become Prime Minister in July 2019 because: 

  • he was the MP for Uxbridge and South Ruislip
  • he had been the leader of the Conservative party since July that year
  • the Conservative party had a Commons majority with the support of a smaller party, the DUP.  

And also because his predecessor, Theresa May, had resigned as Prime Minister when it became clear, having failed to deliver an acceptable Brexit plan, that an insufficient number of Tory MPs supported her remaining in office. She jumped before she was pushed. May had succeeded David Cameron, when he stopped being Prime Minister in July 2016, standing down when he had lost the EU referendum.  

Under Boris Johnson, the Conservatives retained the right to be the government because they won an outright Commons majority of 80 at the election in December 2019. Johnson, however, resigned in September 2022 having been unable to contain a revolt of Tory MPs aghast at his indiscipline in office and following a series of scandals. Liz Truss, elected Tory party leader in succession to Johnson, lasted a mere 44 days, and she too was ousted having proven beyond doubt to be both unpopular and unsuccessful.  

Whenever an election takes place it is the number of Commons seats that you have – having 50 per cent plus 1 at least – which enables a party to form a single-party government, not the percentage of votes you win at the election. For instance, by winning 355 individual constituencies at the 2005 general election, Labour’s 35 per cent of the overall vote meant they won 55 per cent of Commons seats, so had a parliamentary majority of 66. In 2010 the Conservatives, unusually, being the largest party in terms of Commons seats, but lacking enough MPs to form a single-party majority, had to form a coalition with the third-placed Liberal Democrats. And, in 2015 and 2019, the Tories had majorities of 17 and 80 respectively, making Cameron and Johnson Prime Ministers heading single-party governments (May, inheriting Cameron's majority in 2016, lost it at the 2017 election, but remained Prime Minister with the support of that smaller party, the DUP).  

So the Prime Minister can only be Prime Minister because their party elects them as leader and that party secures and thereafter retains a Commons majority at an election (either as a single-party government, which is usually the case, or, if lacking a Commons majority in a hung parliament where no party has a majority over all other parties combined, in coalition with another party or parties). To remain Prime Minister the office holder requires their MPs to allow them to remain as party leader. Usually, the recent defenestrations of Johnson and Truss notwithstanding, government MPs, even should some disagree with certain aspects of their party government’s policy, supply that government after an election and thereafter support its continuation in office until the composition of the Commons changes at the next election. They do so even under a different Prime Minister who emerges within a parliament, not following an election. A new Prime Minister can thus take office within a parliament – in between elections – when an incumbent stands down. This happened in 1990, when John Major replaced Margaret Thatcher; in 2007, when Gordon Brown replaced Tony Blair; in 2016, when Theresa May replaced David Cameron; in 2019, when Boris Johnson replaced Theresa May; and, most remarkably, twice in 2022, when Boris Johnson was replaced by Liz Truss, Truss in turn then being replaced by Rishi Sunak, whom the Tories chose as their new party leader. 

Take the quiz about how other countries vote



CORRECTION: We had originally misplaced Maidenhead in Surrey; we've moved the town back up the A330 to its actual home in Berkshire. 


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