Sir Keir Starmer has resigned as the Labour MP for Holborn and St Pancras, triggering a by-election that poses an immediate challenge for his successor as party leader, Andy Burnham. The move overshadowed Burnham's first appearance before MPs since becoming Prime Minister, and hands an unexpected opening to the Green Party, whose leader Zack Polanski is reportedly poised to stand for the seat. The contest is being watched as an early test of whether Burnham can hold together Labour's traditional urban support against a resurgent Green challenge.

Starmer, who had previously said he intended to stay on as a backbencher until the next general election, said Tuesday was "the right time" to step down, adding he would now focus on international affairs including defence, security, trade and technology. A Green Party spokesperson confirmed Polanski's long-standing interest in the seat, where he has lived for over 20 years, following months of speculation. Analysts noted the constituency's demographics — 66% non-white voters, 51% graduates and 47% aged 16-39 — make it fertile ground for a Labour-to-Green swing, while independent candidate Andrew Feinstein and parties including Reform UK and Restore Britain have also signalled they will contest the seat.