Zack Polanski was selected as the Green Party’s candidate for the Holborn and St Pancras by-election on Friday - and immediately launched a stinging attack on Labour.
The Green leader announced his bid a week ago to be a London MP.
His move came just days after Sir Keir Starmer quit as the MP for the north London constituency.
Mr Polanski faced a surprise challenge from local councillor Hamza Chowdhury for the Green nomination.
But the party leader won overwhelming support from Green Party members in Holborn and St Pancras to fight the contest.
He said: “Holborn and St Pancras, I’m here to fight for you.
“I love this place and so do the people whose doors I’ve knocked on in recent days – but it can be even better.
“I know times are tough in Holborn and St Pancras. My story of constant moving around flats is such a common story for Londoners – and I know many people here are really struggling right now. I’ll fight to turn this around.”
He added: “Greens are underdogs here but we have the momentum – and I know we can achieve a historic victory in one of Labour’s safest seats.
“We will fight a positive campaign on the issues we know people care about, and to put as much pressure as we can on Labour to lower bills, for much more affordable housing and rent controls, to reduce massive wealth inequality and to make sure Labour don’t back down on climate action by allowing more North Sea drilling.”
Mr Polanski will now take on Labour’s Sagal Abdi-Wali, the leader of Camden Council, at the October 8 contest.
Andy Burnham made a surprise visit to north London on Thursday to ramp up Labour’s by-election campaign.
Polls in past months had suggested that the Greens could pull off a shock victory in the previously safe Labour seat.
But the most recent survey suggested, that with Mr Burnham in No10, Labour may hold onto it.
The by-election could provide an early test of Prime Minister Andy Burnham’s popularity with the public.
Labour has held the Holborn and St Pancras seat continuously since it was created in 1983, but voters there have moved towards Green and independent candidates in recent years.
Sir Keir won the London seat with a majority, of 11,572 at the 2024 general election.
He gained 18,884 votes, with the Greens trailing in third place with 4,030.
Independent candidate Andrew Feinstein gained 7,312 votes and came second and he has hinted at a willingness to do a deal with the Greens to deny Labour victory.
The popularity of the independent and Green candidates at the 2024 contest, finishing second and third respectively, may have been partly down to former Labour voters protesting against Sir Keir’s refusal to take a stronger position on Israel’s actions in Gaza in the war with Hamas.
Support for the Greens surged in Mr Polanski’s first year as leader but the party’s national polling has dipped since Mr Burnham replaced Sir Keir in Downing Street.
Only two Labour MPs have ever represented Holborn and St Pancras: former health secretary Frank Dobson, from 1983 to 2015, then Sir Keir.
The Green Party also made gains in the May local elections in the London Borough of Camden, the local authority that covers the constituency.
Government chief whip Anneliese Midgley moved the writ in the House of Commons on Monday, officially triggering the by-election.
Sir Keir revealed his plans to leave Parliament earlier this month, saying he felt it was the “right time” to step down and focus on new work on international affairs
