Polls are open in Holborn and St Pancras, where voters in the north London constituency are electing a new member of parliament to succeed Sir Keir Starmer.
What was once considered a safe Labour seat has turned into one of the most watched contests in recent British politics. Green Party leader Zack Polanski has run a high-energy campaign and is mounting a serious challenge, forcing Labour to fight hard to hold ground it has long taken for granted.
The result is being read as a crunch test for the Labour leadership and a measure of how far the Greens can convert protest energy into parliamentary seats. Turnout in the central London seat will be closely scrutinized once counting begins.
Polling stations are open throughout the day, with live coverage tracking turnout and early indications through the evening. A Green victory would send shockwaves through Westminster; a Labour hold would steady nerves after weeks of difficult headlines.
