Sir Keir Starmer has said the Greens' victory in the Gorton and Denton by-election is "very disappointing" - but he has vowed to fight on.
Hannah Spencer has become the Green Party's fifth MP in parliament, after beating Reform UK, who finished second, and Labour, who came third, to the seat.
The Greens received 14,980 votes to claim a majority of more than 4,400 votes. Reform picked up 10,578 votes, while Labour received 9,364 votes.
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It is the Greens' first-ever victory in a parliamentary by-election, and the first time the party has won a seat in the north of England.
'Only Labour can unite our country'
Reacting to the result, the prime minister said: "It's a very disappointing result. Incumbent governments quite often get results like that mid-term.
"But I do understand voters are frustrated, they're impatient for change."
He added: "I will keep on fighting for those people for as long as I've got breath in my body. I will also fight against the extremes in politics on the right and on the left - parties who want to tear our country apart.
"The Labour Party is the only party that can unite our country and our communities."
Speaking to Sky News following her victory, Ms Spencer said she "knew it was always possible" for the Greens to win in Gorton and Denton.
"There is an appetite here for change," the 34-year-old said.
"There are people across this constituency and much further beyond who are rejecting the old political parties and who are coming together to fight for something better, but who are doing it positively and in a really hopeful way."
Green Party leader Zack Polanski has called Ms Spencer's victory "an incredibly important win" and said "this will transform the face of British politics".
Speaking to Sky News, he added: "It's important to say this was the Green Party's 127th target seat. We've won this.
"It means that the Green Party vote is never a wasted vote. There's no no-go areas right across England and Wales. You can expect to see more Green Party councillors and more green MPs."
He insisted the Greens are "the only party joining the dots and putting all these things together", citing the party's policies on the cost of living, the war in Gaza, and environmental policies.
But he also criticised Labour for what he called "a shameful, dirty campaign - spreading lies about Green policies and even faking a tactical voting website".
"They knew they couldn't win, but they risked splitting the vote and letting Reform in," he said.
The turnout for the vote was 47.6% of the electorate, slightly lower than the 47.8% at the 2024 general election.
Nigel Farage has reported claims of "family voting" to the police and the Electoral Commission, the election watchdog.
It follows concerns raised by election observer group Democracy Volunteers. The organisation said it witnessed record numbers of illegal "family voting".
Family voting is where two voters either confer, collude or direct each other on voting, which is illegal under the 2023 Ballot Secrecy Act.
The Reform UK leader warned: "What was witnessed yesterday is deeply concerning and raises serious questions about the integrity of the democratic process in predominantly Muslim areas.
"If action isn't taken now, then we will ensure it is after the next general election."
Read more on the by-election:
By-election result 'bruising' - but replacing PM 'won't solve problem'
Explained: Who is Hannah Spencer?
Analysis: This is a parliamentary result for the ages
Greater Manchester Police has confirmed it has received a report. In a statement, a spokesperson for the force said: "We are in the process of reviewing this report and will provide a further update in due course."
The Electoral Commission has also confirmed they received a letter from Mr Farage - but pointed out the organisation "does not have a direct role in investigating allegations of electoral fraud" as this is a "matter for the police".
'A tough result for our movement', says PM
Home Secretary Shabana Mahmood's immigration policies have been cited by some in Labour as a key reason for the party's loss of a seat it has historically held, which was also its 38th safest seat in 2024.
Former deputy leader of the Labour Party, Angela Rayner, has said the result "must be a wake up call" and urged the government to "really listen - to reflect".
One Labour MP told Sky News' deputy political editor Sam Coates that Sir Keir should resign following the loss, while a number of Labour backbenchers have publicly called for "change at the top", either in the form of the PM's departure or a different direction.
Sir Keir has written to all Labour MPs on Friday morning, recognising that it is a "tough result for our movement".
He said the Greens won because they were "able to capitalise on an endorsement from George Galloway to win over enough voters to push them over the line".
The PM added the Green Party is "unstable" and "cannot survive a general election campaign".
"They simply do not have the resources, the activist base or the local knowledge to replicate this victory across the country," he said.
He also hit out at Ms Spencer, saying her constituents "now have a representative who is more interested in dividing people than uniting them".
"We have to learn lessons from that, and we will," he insisted
(c) Sky News 2026: Starmer says by-election loss 'very disappointing' - but vows to fight on


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