Camden Council leader Sagal Abdi-Wali has retained the Holborn and St Pancras seat for Labour in a by-election called after former prime minister Sir Keir Starmer stepped down, defeating Green Party leader Zack Polanski.
Ms Abdi-Wali won with 12,292 votes or 45%, with Mr Polanski in second with 8,928 or 33%.
Labour secured a majority of 3,364 with a 13% swing from Labour to the Greens.
Conservative candidate Ewan Cameron came in third, followed by Reform and the Lib Dems.
The by-election came in a week that has seen Mr Polanski under pressure after Green members backed a motion declaring Zionism is racism, but he insisted he would stay on as leader.
A former Green leader earlier suggested Mr Polanski could face a threat to his leadership if he lost.
Caroline Lucas, the three-time former Green Party leader who was its first MP said people should “see what happens in the coming days and weeks” when asked about his future on BBC Newsnight.
After the result, when asked by Sky News if he would continue as leader, Mr Polanski said: “Yes absolutely, that’s what leaders do.”
He said it had not been a mistake fighting for the seat, saying he was happy to stand in elections.
“I don’t think it was a mistake at all, I think it has been an opportunity to present Green Party values, to talk about rent controls, tackling the climate crisis, nationalising Thames Water, it’s been amazing to speak to people in Holborn and St Pancras, I’ve really enjoyed that.”
The north London seat has been a Labour stronghold since its formation in 1983, held by Frank Dobson for 32 years and Sir Keir for 11.
The Greens had conceded ahead of the official declaration but said the “huge swing” was an “incredible result”.
A Green Party spokesperson said: “We are disappointed to miss out tonight but any ideas of a return to the cosy two-party system is just wishful thinking on the part of the establishment. We are here to stay and just getting started.
“The voters have delivered a message to the Labour Government today. They want real change; they want rent controls, they want to see an end to the privatised water rip-off; and they want to see justice for Palestinians.
“On the climate crisis, Labour will not have an easy political ride if they bring in new North Sea drilling.”
Ms Abdi-Wali said her victory was a testament to the success of diversity in the borough.
“I hope this moment also says something about who gets to belong, for people who look like me, I hope they see themselves reflected in this place and know that their path too belongs here. This is Camden, and this is Britain.”
She added: “I’ve watched people across Camden work to build communities where difference is not something to fear, but something to value. We cannot allow that work, built over generations, to be casually undone by those who seek to divide us.
“We have a responsibility to defend what we’ve built, and pass it on. Hope is not naive. Without hope there is nothing to build on, and nothing to build for.
“People need to believe that politics can make their lives better and that their communities can thrive and that tomorrow can be better than today.”
She added: “That is what this victory represents. Winning not by making people afraid of each other, but by bringing people together.”
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