Sir Keir Starmer’s Labour party never promised much excitement. What it did seem to offer was stability and grown-up government, sorely lacking under recent Conservative predecessors. This week’s self-generated crisis over the prime minister’s leadership and a startling Budget U-turn have shredded Labour’s claim to quiet competence. They ratchet up the jeopardy around a Budget that was already crucial to the government’s fortunes and those of the country.

On Wednesday, bemused Britons woke to reports that the unpopular prime minister was vowing to fight off any challenge to his leadership. Unnamed briefers pointed a finger at health secretary Wes Streeting, one of his more capable ministers. Streeting was forced to deny traitorous intent, and decry efforts to “kneecap” him by a “toxic” culture in Downing Street.

By Friday, the headlines were dominated by news first reported by the Financial Times that the government had scrapped a Budget plan to break its manifesto promise and raise income tax rates — for which it had spent weeks preparing the ground. The volte-face was explained in part by an improved forecast from the government’s fiscal watchdog, putting an expected ￡30bn hole in the public finances at nearer ￡20bn. But the move also reflected fears that the tax plan would further alienate voters and fractious Labour MPs. Sterling and the gilt markets reacted badly.