UK prime ministers today are about as secure in their jobs as football managers. In the nearly three decades between 1979 and 2005, Britain had just three prime ministers: Margaret Thatcher, John Major and Tony Blair. From 2005-2015, we again had three: Blair, Gordon Brown and David Cameron.

But from then on we have had no less than six: Cameron, Theresa May, Boris Johnson, Liz Truss, Rishi Sunak and now Keir Starmer, currently fighting to retain his job and explaining to parliament why he supported Peter Mandelson for a key ambassadorship.

Prime ministers usually resign after serious political failures – from May failing to get a Brexit deal through parliament after losing her working majority, to a series of scandals under Johnson leading to a mass resignation of his ministers. Truss was evicted in record time after her mini-budget, proposing large unfunded tax cuts, crashed the financial markets.

But prime ministers have always faced problems in the past and survived, so why is it proving fatal to their incumbency now?

One explanation is that it reflects the increasing fragmentation of the British party system. In the 1987 general election, Labour and the Conservatives took 73% of the total vote. In the 2024 election, they took 57%. In particular, when prime ministers can’t get a working majority, something which happened in 2010 and again in 2017, they are quick to be blamed by their supporters when things go wrong.

Another explanation is that while all prime ministers........

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