Londoners are voting in a closely watched by-election in the Holborn and St Pancras constituency, where the Green Party is threatening the position of Labour under Prime Minister Andy Burnham.
Zack Polanski, leader of the left-wing Green Party of England and Wales, is among the candidates contesting the parliamentary seat, which was left vacant after former prime minister Keir Starmer resigned.
Labour has held the seat continuously since the constituency was created in 1983. Thursday's vote is widely seen as a two-horse race with the Greens, with thousands of campaigners from both parties knocking on doors in recent weeks.
The ballot is viewed as the first test of whether Burnham can rebuild voters' trust in Labour after the party saw its support among left-leaning voters fall over the war in Gaza.
The by-election also gives him a chance to halt the momentum of the Greens under Polanski, 43, who describes himself as an "eco-populist" and became party leader last September.
Green gains in parts of London contributed to Labour's grim local election results in May, setting in motion the process that led to Starmer's resignation on 20 July and his replacement by Burnham.
Since then, Burnham has adopted a tougher stance on Israel and sought to blunt the Greens' rise by setting out a more left-wing vision for Britain.
Polanski, standing for the Greens, faces Labour's Sagal Abdi-Wali, a local council leader who came to Britain as a child refugee from Somalia in the 1990s and is tipped to win.
Polling stations are due to close at 10pm local time (21:00 GMT), with the result expected in the early hours of Friday.
Other candidates include: Barmy Brunch of the Official Monster Raving Loony Party; Ewan Cameron, Conservative; independent Tom Darwood; Clare Fischer of the Climate Party; George Galloway of the Workers Party; Tyler Johnson of Restore Britain; Briony Anne Lonbay Kapoor of Rejoin EU; Peter Newman of Reform UK; Ketankumar Pipaliya, UK VOICE; Jonathan Silberman of the Communist League; Helen Spiby-Vann of the Christian Peoples Alliance; Patrick Thomas Stillman, Liberal Democrat; and Ben Walker, UKIP.
