Keir Starmer’s resignation would make his successor Britain’s seventh prime minister since the 2016 Brexit vote, a striking measure of political churn in a system once associated with stable parliamentary government.

The sequence is familiar but still extraordinary: David Cameron, Theresa May, Boris Johnson, Liz Truss, Rishi Sunak, Keir Starmer and now the expected next Labour leader. The rotation cuts across parties, suggesting the problem is deeper than any one leader’s style.

Brexit is part of the explanation, but not the whole of it. The referendum reordered party coalitions, hardened arguments over sovereignty and weakened the ability of prime ministers to hold together broad governing mandates.

Economic constraint has done the rest. Prime ministers have faced high public expectations alongside limited fiscal room, pressure on health and social care, defence demands, housing stress and low trust in institutions.

Party rules have also made leadership change easier than national electoral renewal. A governing party can replace its leader without a general election, but each internal transition raises a mandate problem: voters chose a party and programme, not necessarily the next occupant of Downing Street.

Starmer’s departure therefore matters beyond Labour. It points to a political system where majorities can be large, but authority can still be brittle.

The question for Andy Burnham, if he succeeds Starmer, is whether he can turn another leadership reset into durable governing capacity. Recent history suggests that changing the person at the top is the easier part.