Labour was preparing for a massive battle against Zack Polanski’s Green Party at a London by-election after Sir Keir Starmer announced he was quitting as an MP.
Green leader Mr Polanski is considering standing in Holborn and St Pancras to get into Parliament.
But Labour was defiant over holding onto the seat.
“They can fancy their chances all they like, but we will be fighting hard and we'll be making sure that we win,” said Cabinet Office minister Sally Jameson.
Labour won the seat with a majority, of 11,572 at the 2024 general election.
Sir Keir’s party gained 18,884 votes, with the Greens trailing in third place with 4,030.
So it would be a gamble by Mr Polanski to risk a humiliating defeat if he did decide to stand.
The former Prime Minister revealed he was quitting as the local MP on Tuesday, saying he felt it was the “right time” to step down from the Commons and focus on new work on international affairs.
The looming contest in the north London constituency, which Sir Keir has represented for 11 years, presents a major test of Andy Burnham's leadership.
The poll will come just months after the Greens made historic gains in London at May's local elections, seizing control of a string of councils at the expense of Labour.
Rob Ford, Professor in Politics at Manchester University, wrote on X: "The seat has exceptionally high shares of non-white voters (66 per cent), graduates (51 per cent) and young ppl (47 per cent aged 16-39) – all groups that have swung Lab to Green in recent years.
"This could be a humdinger of a by-election."
A further twist could come in the shape of Andrew Feinstein, an independent who gained 7,312 votes and came second in Holborn and St Pancras at the last election.
Hinting at a willingness to do a deal with the Greens to deny Labour victory, he posted: "We must ensure that all progressive forces in Holborn & St. Pancras unite in order to deliver an MP who represents us & will work for us.
"An MP who is the antithesis of everything that Starmer stands for."
Sir Keir was replaced by Mr Burnham as Prime Minister and Labour leader in the summer after losing the support of his party in the wake of dire local election results.
He told the Camden New Journal: “It has been a huge honour and privilege to represent the best constituency in the country and to have taken over from Frank Dobson, who was such a great constituency MP and a member of the last Labour government.
It has been an honour and a privilege to represent Holborn and St Pancras for the last 11 years.
— Keir Starmer (@Keir_Starmer) September 1, 2026
My statement. pic.twitter.com/xzu4xcIno2
“I will now focus on other things and that will be international affairs including defence, security, trade and technology in a fast-changing world.
“I will also continue, as I have done for a long time now with others, work on combating violence against women and girls; that is an issue that matters to me hugely. It’s work I started before I became an MP and it’s work I’m keen to continue after I’m an MP.”
Reflecting on his time as Prime Minister, Sir Keir again pointed to Labour’s 2024 election victory as a huge achievement as he thanked the Holborn & St Pancras Constituency Labour Party for its support during his time as party leader.
Mr Burnham paid tribute to his predecessor as he spoke from the Commons despatch box as Prime Minister for the first time on Tuesday afternoon.
He said: “(Sir Keir) has given this country a lifetime of public service, particularly for those fighting for justice.
“When (Sir Keir) delivered in this House on his promise to the Hillsborough families and everyone involved in the Hillsborough Law campaign, it was a moment I will never forget.”
He said Sir Keir’s leadership took the Labour Party from “one of its worst defeats” to one “its best victories”, adding that his predecessor had “created a platform to change this country for the better”.
Conservative Party chairman Kevin Hollinrake said: “Whilst we have disagreed with almost all of Sir Keir Starmer’s decisions, I wish him and his family well for the future.”
But he added: “It is fitting that his final act as a Member of Parliament is yet another U-turn having pledged to stay on.”
