For much of the post-war era, Britain was known for prime ministers who lasted. Once a leader reached Downing Street, they were expected to stay there. The dominance of two established parties, relatively disciplined parliamentary blocs and a first-past-the-post electoral system that often turned votes into workable Commons majorities all helped give prime ministers a stable base. Margaret Thatcher and Tony Blair both got something that now seems almost unimaginable: a decade in power.

But British prime ministers are now coming and going in quick succession. The country is on course for its seventh prime minister in a decade. Theresa May and Boris Johnson each lasted just over three years. Liz Truss managed only 49 days. Keir Starmer was supposed to be different: he entered Downing Street after Labour won a landslide majority in 2024. Yet he, too, is now leaving after barely two years. Why has Britain’s famed stability given way so quickly to political chaos?

There are several obvious explanations, but none is enough on its own. Has social media helped harden political divisions? Almost certainly. But Britain is hardly the only country with the internet. Has Brexit made the country harder to govern? Yes. It cut across party lines, deepened political identities and left prime ministers managing not just policy disputes but rival ideas of what the country should be. Yet, as academics have pointed out, Brexit did not create Britain’s instability out of nowhere. It accelerated pressures that were already building inside the political system.

Could it simply be that Britain has had a run of bad leaders? As I like to remind my students, some people are just no good at the top job. For some recent prime ministers, the problem was competence. Theresa May could not get her Brexit deal through Parliament, while Liz Truss’s radical economic experiment collapsed almost as soon as it began. For others, it was judgement and ethics. Boris Johnson broke the rules while asking the country to follow them, then damaged himself further by denying what had happened. Keir Starmer managed to blur the categories: his government was marked by policy indecision, but also by serious errors of judgement, most notably the appointment of Peter Mandelson.

But bad leadership only takes us so far. Britain has had plenty of poor and failed politicians before. The deeper problem lies in the changing relationship between prime ministers and their own MPs. Any prime minister needs their parliamentary party to vote through their programme and defend them when trouble comes. For much of the post-war era, that relationship was more reliable. Since the 1970s, however, MPs have become more willing to rebel against their own parties, challenge their leaders and, when necessary, help remove them. To borrow the political scientist George Jones’s famous image, a prime minister’s power is like an elastic band. It can stretch, but only so far.

The fraying relationship between MPs and prime ministers is behind many of the major events in British politics since the 1990s. Iraq badly damaged Blair’s authority with much of his own party. In 2003, so many Labour MPs rebelled against his Iraq policy that Blair and those around him feared it could cost him his premiership. The rebellion failed, but the war and its consequences drove a lasting wedge between Blair and many of his MPs. David Cameron held a Brexit referendum because his own rebellious MPs, long hostile to Europe, kept pushing the issue. When voters chose Leave, he resigned. Boris Johnson’s Partygate lies proved fatal when his own MPs refused to back him. Keir Starmer’s welfare cuts and harsh immigration policies forced his own Labour MPs to decide between loyalty and principle.

This fraying has made MPs more willing to move against their leaders. Removing prime ministers between general elections is now a modern British habit. The last prime minister both to enter Downing Street after winning a general election and to leave after losing one was Edward Heath in 1974. Since then, leaders have more often been brought down by internal party pressure, scandal, resignation or succession than by voters directly ending their premiership at the ballot box. The habit is speeding up. Of the last five prime ministers, four have left after pressure from inside their own parties, while only Rishi Sunak was removed by voters in a general election.

One final factor has added to the chaos: voters are changing. Britain is no longer a strong two-party system. In England, voters are now splitting between several parties, not lining up behind Labour and the Conservatives as reliably as they once did. In Scotland, the divide over independence still shapes politics. In Northern Ireland, elections follow a different party system, shaped by unionism, nationalism and the growing centre ground. In Wales, Labour now faces a stronger challenge from Plaid Cymru and Reform.

This new voting landscape makes life harder for both prime ministers and MPs. For leaders, winning no longer means simply holding together Labour or Conservative voters. It means deciding which voters to chase, which promises to soften and which parts of the party’s coalition can be risked. Keir Starmer’s inner circle appeared to believe that tougher immigration policies could hold or win back voters drifting towards Reform. But those policies angered Labour MPs and created more space on Labour’s left, where the Green Party was already showing it could take votes and seats from Labour.

Fragmenting voting patterns also make sitting MPs more vulnerable. When voters are less loyal and old party loyalties are weaker, MPs have more reason to panic when their leader becomes unpopular, reckless or scandal-hit. Instead of waiting for voters to pass judgement at the next general election, they have an incentive to act first. That makes leaders easier to remove and prime ministers quicker to replace.

Taken together, weak leaders, restless MPs and fragmenting voting patterns have created a self-reinforcing cycle. Each failed premiership makes the next one harder. A new prime minister arrives promising a reset, but inherits the same deep problems, the same anxious MPs and even less patience from the public. Instead of restoring stability, each change of leader makes the next prime minister easier to bring down.

That is the cycle Andy Burnham would inherit, and the question is whether he can break it. Burnham, who was Mayor of Greater Manchester from 2017 until his return to parliament, arrives with a reputation among his supporters for getting things done and for explaining his politics in plain language. The Makerfield by-election, where he won a seat to return to parliament, gave them some evidence for that claim: Labour increased its vote there, bucking the wider trend of fragmented politics.

But there is still much we do not know. Not everyone is convinced that Burnham achieved as much in Greater Manchester as his supporters claim. Public control of transport gave him a powerful story to tell, but national government would test those promises much harder. If he keeps Labour’s harsh immigration policies, if his promises to restore public control prove thinner than voters expect, or if his popularity begins to fall, goodwill inside the party could evaporate quickly. Burnham would then face the same danger as his recent predecessors: MPs deciding that their leader has become a risk they cannot afford. That would put him back inside the cycle he was supposed to break.

The views expressed in this article are the author’s own and do not necessarily reflect Al Jazeera’s editorial policy.