Asked the above question on BBC Breakfast, Transport Secretary Heidi Alexander says Labour's National Executive Committee decided Andy Burnham should not run.

She says this is because he is the mayor of Greater Manchester - and running a by-election to replace him would be equivalent to running 20 parliamentary by-elections.

She adds that speaking about Burnham does a disservice to Angeliki Stogia - who fought the Gorton and Denton by-election for Labour. She "threw her heart and soul" into the campaign, Alexander says.

The minister adds that the "anti-Reform" vote went to the Greens this time round - but that doesn't necessarily mean it will happen again.

Alexander says it's her view that Labour remains the party to stop "the toxicity and division of Reform".

The minister out on the government's media round this morning is Transport Secretary Heidi Alexander, who's speaking now on BBC Breakfast.

She acknowledges that it's been a "difficult and disappointing night" for Labour - but that it's not uncommon for governing parties to lose a mid term by-election. People should not "over-interpret" this result, she adds.

The result does not mean that the majority of people in the country want Zack Polanksi or Nigel Farage to be prime minister, Alexander says, adding that the circumstances leading to the by-election had been challenging for Labour.

Labour will reflect carefully, she continues, but "I don't think we should read too much into it due to the different dynamics at play during a parliamentary by-election".

Recent by-elections show just how fragmented British politics has becomepublished at 07:58 GMT

07:58 GMT

Henry Zeffman
Chief political correspondent

Image source, PA Media

This isn’t the two-party double act we are all used to, but something much more fragmented.

It’s worth remembering that this is the second by-election since the 2024 general election. Last May in Runcorn and Helsby, Labour were also beaten by a non-traditional insurgent force: in that case, Reform.

Yes, it is fairly typical for governing parties to get a kicking in by-elections. But taken together, these by-elections point to a new dynamic of surging political forces to the traditional parties’ left and right.

It’s worth remembering too the Caerphilly Senedd by-election in October where Labour were usurped by both Plaid Cymru and Reform.

Labour MPs have long digested the possibility that their main rival to the right come the next general election might be Reform UK, not the Conservatives.

In response, they hoped to become the anti-Reform party, uniting a broad front of voters who do not want Nigel Farage to become prime minister, even if many of those voters do not much like Keir Starmer and are voting tactically.

But what if some of those voters come to see the Greens, instead, as their best hope of stopping Reform? After all, that’s the decision voters here in Gorton and Denton made.

We've got a final few lines to bring you from Zack Polanski's BBC Breakfast interview.

Asked about accusations of family voting at the Gorton and Denton by-election, the Green Party leader says if any wrongdoing has occurred then it should be investigated.

But he adds that Manchester City Council has been robust in saying there was no irregular findings.

Family voting is an offence that can be committed when a person casting a ballot in an election is joined in a polling booth by someone who intends to influence how they vote.

As we reported earlier, an election observer group claimed its team witnessed "concerningly high levels" of such voting on Thursday. Manchester City Council, whose returning officer oversaw the election, disputed the claim - saying "no such issues" were reported.

The Green Party leader has just referenced an "absolutely huge swing" between the parties in the by-election overnight.

And indeed, Labour's share of the vote fell from 51% to 25%, dropping by just over 25 percentage points.

Meanwhile, the Greens increased theirs from 13% to 41% - that's about 28 percentage points in the other direction.

You can see the change - or swing - clearly in the graphic below:

Back to Polanski on BBC Breakfast - he calls the result in Gorton and Denton an "absolutely huge swing" for the Greens.

"I think it's undoubtedly a moment in British politics that has the potential to transform the faith of British politics," he says.

On Reform UK - who came second - Polanski says he would've been "really worried" if the party had won, branding their candidate Matt Goodwin "extreme".

On Labour, he says the governing party lost the seat for reasons that are "quite obvious", claiming the public are have been "disappointed" since the 2024 election.

Image source, Reuters

Labour MP Richard Burgon is reacting to the Greens's by-election win in Gorton and Denton - a seat previously held by Labour.

Writing on social media, he says "blame for Labour’s defeat lies squarely with Keir Starmer and his clique".

Burgon continues, saying on social media "they put factional interests over having the candidate best placed to win, Andy Burnham".

He adds that if Labour is to be the "stop Reform" party then its leadership must "stop treating progressive voters with contempt - and start appealing to them".

For Labour, this is the kind of result that nightmares are made ofpublished at 07:31 GMT

07:31 GMT

Chris Mason
Political editor

Finishing third, with an outcome that makes real the long-standing fear of some in the party, of facing an insurgency not just broadly from the right, from Reform UK, but from the left too, with the Greens.

The prime minister is likely to wake up with both a tactical and a strategic question in his mind.

Firstly, should he have allowed the Mayor of Greater Manchester Andy Burnham to be Labour’s candidate?

Burnham’s supporters in the count centre told me they do think he would have made a significant difference.

And secondly, has Labour’s focus on Reform alienated some of the party's natural supporters? The inquest among Labour MPs began in the middle of the night.

"Keir needs to own this. He's out of chances," one texted me.

A final thought about the Green Party of England and Wales: remoulded, re-energised under Zack Polanski, they’ve converted buoyant opinion polling into a breakthrough by election victory, which will lift their mood as much as it depresses Labour’s.

People are ready for an alternative to Labour, Green Party leader Zack Polanski tells BBC Breakfast.

He says the same people are also rejecting the divisive politics of Reform UK.

There are now "no no-go areas for the Green Party in England and Wales", Polanski says, adding the party intends to get out into as many communities as it can.

• A reminder of the final vote count: The Greens won the Gorton and Denton by-election with 14,980 votes; Reform UK came second with 10,578; Labour came third with 9,364 and the Tories finished fourth with 706.

Fresh from his party's first-ever Westminster by-election win, Green Party leader Zack Polanski is due on BBC Breakfast shortly.

We'll bring you key lines from what he has to say right here.

Image source, Getty Images

"I said we were here to replace Labour and I meant it," says Green Party leader Zack Polanski, following his party's win.

Reform UKleader Nigel Farage claims his party's loss is a "victory for sectarian voting and cheating", and that Reform offered a "great" candidate in Matt Goodwin.

• As a reminder, election observers said they witnessed "concerningly high" levels of "family voting" at polling stations - a claim disputed by the council

Labour Party chair Anna Turley calls the result "clearly disappointing", after the party's deputy leader Lucy Powell says the Greens "turned out their vote more than we have".

A spokesperson for the Conservatives - who finished fourth - say the result shows Keir Starmer has "killed the Labour Party".

We're yet to spot any reaction from the Liberal Democrats, who finished fifth.

This is a dismal result for Labourpublished at 07:11 GMT

07:11 GMT

Henry Zeffman
Chief political correspondent

Image source, Reuters

This is a dismal result for Labour and will inevitably become part of the discussion in the party about Sir Keir Starmer’s leadership.

The prime minister may well face criticism for the decision, on which he spent personal political capital, to block Andy Burnham from standing.

But what will probably be the most intense subject of debate within Labour will be the question of what this means the party’s political strategy should be going forwards.

One senior figure on the ‘soft’ left of the Labour Party, a faction associated with figures like the former deputy prime minister Angela Rayner, said to me: “This must be the end of the McSweeney strategy of alienating our own voters.”

To translate: now that Morgan McSweeney, the prime minister’s chief of staff and main strategist, has left, Starmer should become more left wing.

Another striking element of the result is that actually Labour figures close to the campaign here in Manchester had been cautiously optimistic in recent days. They were wrong… by some distance.

One Labour MP said to me: “The campaign was great. But you can’t have an organisational solution to a leadership problem.”

It's been a big night for British politics - but what does the Green Party's victory mean for each of the main parties going forward?

Newscast's Adam Fleming, the BBC's political editor Chris Mason, and political and investigations correspondent Joe Pike are discussing it live.

• Watch the special episode of Newscast live at the top of this page or, alternatively, listen on BBC Sounds

The Green Party has won a by-election for the first time ever, with Reform UK finishing in second.

Labour, the party of government, who had held the seat came third.

Our political editor Chris Mason says Keir Starmer has spent much of his time as prime minister fretting about the rise of one insurgency, Reform UK, broadly on the right.

Now, Chris says, Starmer has to fear the reality of an insurgency broadly on the left - from the Green Party.

Watch his snap analysis from Manchester below and read his analysis here:

Green Party's historic success means future of British politics is more uncertain than everpublished at 06:25 GMT

06:25 GMT

Professor Sir John Curtice
Polling expert

Image source, EPA

Hitherto, the Greens had never won more than 10% of the vote in a parliamentary by-election, a figure they reached in the Somerton and Frome by-election in 2023.

Now Hannah Spencer has already claimed her place in the history books as the first ever Green candidate to win a parliamentary by-election.

Not only did Spencer win, but she won well, significantly outperforming the expectations of the polls.

Rather than winning narrowly, the party won 40.7%, enough to put them as much as 12 points ahead of second placed Reform. It represented as much as a 27.5 point increase on the party's share in 2024.

Labour, who had not hitherto lost an election in the area since 1931, fell into third place. The party's 25.4% of the vote represented a near halving of their 50.8% of the vote in 2024 and the 13th biggest ever fall in the party's support in a by-election.

Meanwhile, the Conservatives lost their deposit with just 1.9% of the vote, their worst ever by-election result.

Apart from the exceptional circumstances of the Rochdale by-election in 2024, when Labour disowned their candidate, leaving George Galloway to defeat a second-placed independent candidate, it is the first time that neither Labour nor the Conservatives have been one of the top two parties in a by-election contest.

With the two parties both running at 20% or less in the polls, the Conservative-Labour duopoly that has long dominated post-war British politics has never looked weaker.

The Conservatives are struggling to fend off the challenge from Reform, and now, at the other end of the spectrum, Labour's traditional position of the principal party of the left of British politics is evidently under threat from the Greens.

Doubtless the result will raise fresh questions in Labour MPs minds as to whether Keir Starmer should remain prime minister.

• Continue reading my analysis here

Image source, PA Media

Hannah Spencer has made history as the first Green Party candidate to win a Westminster by-election - as well as becoming the party's first MP in northern England.

The 34-year-old currently leads the Greens on Trafford Council, where she has represented the Hale ward since May 2023.

She has previously said she's worked as a plumber since she was 16, and during her victory speech added she qualified as a plasterer two weeks ago.

"I didn't grow up wanting to be a politician. I am a plumber," she said during her speech. "I am no different to every single person here in this constituency. I work hard. That is what we do."

Spencer also stood in the 2024 Greater Manchester mayoral election, and is the owner of four greyhounds - who she took on the campaign trail.

Image source, PA Media

Image source, EPA

The Green Party's Hannah Spencer has won the Gorton and Denton by-election, becoming the first Green candidate to win a Westminster by-election - as well as the party's first ever MP in northern England.

• Spencer won 14,980 votes, making up nearly 41% of the votes cast. Reform UK's Matt Goodwin came second, with 10,578, while Labour's Angeliki Stogia came third with 9,364
• In her victory speech, Spencer vowed to fight for working people, while calling out "divisive figures" who she said "scapegoat" others
• The 34-year-old plumber also apologised to customers who have booked her as "I'm heading to Parliament"
• Labour Party chair Anna Turley called the result "clearly disappointing" - BBC election expert Sir John Curtice says the result is going to make life for Keir Starmer "even worse"
• Reform's Matt Goodwin says his party "embarrassed Labour" but also hit out at the Greens, accusing them of "riding a dangerous wave"
• Earlier, election observers say they witnessed "concerningly high" levels of "family voting" at polling stations - a claim disputed by the council

A spokesperson for the Conservatives - the party came fourth with 706 votes - says Keir Starmer has "killed the Labour Party".

The spokesperson adds: "In losing one of Labour's safest seats, in a constituency that has returned Labour MPs for almost a century, Starmer has shown he no longer commands the support of Labour voters and is now a lame duck leader.

"This result shows the Labour government now lacks any support in the country and has no mandate for the terrible policies it is pursuing - increasing the benefits bill, raising taxes, weakening our borders and the armed forces.

"Only the Conservatives have the experience, the plans and the team to ensure a stronger economy and a stronger country."

Image source, EPA

Green leader Zack Polanski says "almost half" of Labour's 2024 general election voters "abandoned them", as "many switched to voting Green".

"The Green Party saw a record-breaking swing in our direction and more than tripled our vote," he says.

He adds: “If we see a swing like this at the next general election, there will be a tidal wave of new Green MPs.

"When I was elected leader of the Greens I said we were here to replace Labour and I meant it. Hannah was a fantastic candidate and I know she’ll make a brilliant MP."

He also accuses Labour of fighting "a shameful, dirty campaign".

"They knew they couldn’t win, but they risked splitting the vote and letting Reform in," he says, adding: “People everywhere will now know that voting Green is the way to defeat Reform."

Image source, PA Media

Labour candidate Angeliki Stogia declined to speak to the media as she left the count, say reporters in Manchester.

Meanwhile, Labour Party chair Anna Turley has called tonight's result "clearly disappointing".

The constituency had historically been considered a Labour safe seat.

Turley says: "By-elections are normally difficult for the party of government, and this election was no different.

"We have had thousands of conversations over the last few weeks and we know the majority of voters here did not want the poisonous politics of Nigel Farage and Reform.

"We will continue to deliver a programme for government that tackles the cost of living crisis families are facing, creates opportunities for young people and invests in our public services.

"The politics of anger and easy answers offered by the Greens and Reform won't deliver this.

"We will move forwards with a relentless focus on delivering the renewal communities across Britain want to see."