Green by-election win is worst result for Labour and Sir Keir Starmer, writes Natasha Clark

There'll be a lot of soul-searching in Labour in the next few days over how Gorton and Denton went from a safe Labour seat, to being pushed into third place by the Greens and Reform.

We know that unhappiness with Labour has been growing, nationally, for months.

But this shows just the extent that that unhappiness is having on people that Labour have, and should, always be able to rely on to back the red team.

Huge chunks of their key support is now haemorrhaging to the left, after months of focus on how votes have been collapsing on the right.

The PM and his team threw the book at it this week - with a string of briefings that only Labour could beat Reform, optimistic political expectation that it might be better than thought, and even a visit from the prime minister himself.

That's not something you'd expect unless their internal projections thought they had a slither of hope they could hang onto it.

All the Cabinet had been marched up to the area to door-knock and canvass at least twice before the vote.

And what they were reporting back was more positive than I'd expected.

Was that bluster, or just putting on a brave face?

There is another world we could have been waking up to today - Andy Burnham makes his return to Westminster and holds key seat for Labour, bucking national trend.

His supporters think we’d be looking at a very different result this morning, if he’d been allowed to stand.

Today the question for Labour and for Sir Keir will be - was that the right call?

Already this morning, people are re-sharing a social media post from January when critics said having him in Westminster would cause yet more psychodrama.

He simply replied: "I’m not sure losing a by-election does us any good either".

Labour’s defence this morning will be a few messages.

Governing is hard, by-elections are different, this result won't be replicated at a general election, change is coming, and we understand frustrations out there.

But in the same way that we have talked for many months about the threat from Reform, the hard proof of this stunning result will force Labour and Downing Street to rip up their election strategy of solely addressing Reform as their opposition.

Not just once, but twice, the anti-Reform coalition have managed to deny Nigel Farage a seat in Parliament.

It wasn't Labour which achieved that, but Plaid and now the Greens.

Labour will have to spend even more time on addressing those concerns going forward if they want to hold onto their electoral coalition.

With advisers at the top of Downing Street like Tim Allen and Morgan McSweeney gone, the pressure to focus political strategy and messaging solely on Reform will now fade.

The Wes Streetings and Shabana Mahmoods of the party could be on the backfoot - as the Socialist Campaign Group and other Labour backbenchers say a shift to the left is now the only answer.

The Greens are positioning themselves as the country's left wing populist party, in a campaign which didn't focus on what we may have previously thought of as Green issues - environment, nuclear, drugs, to name a few - but public services, jobs and the cost of living.

And as for the Conservatives, losing their deposit? Well, another nail in the coffin for the two-party political system there.

Two relative outsiders from two formerly fringe parties - Matt Goodwin and Hannah Spencer - were able to make themselves the frontrunners, and the number one and two winners.

Yesterday, this was a result no one could predict, with three political parties in the mix.In the end, it wasn't even close, with the Greens landing a whopping 4,000 majority for Spencer.

And as for Sir Keir's future - I don't think he'll be packing up boxes in Downing Street this weekend, or for the weeks to come.

But we will return a question of his personal judgement at the decision to block Burnham from standing in an attempt to see off challenges for longer.

We also may see a boost for potential left-wing challengers like Angela Rayner, after seeing the proof that a Northern, working class woman can beat Reform.

Losing such a heartlands area for Labour will further weaken the authority of the prime minister, making it another signal that it's all a matter of when, not if, he'll be on the way out.