Thelma Walker, former Labour MP for Colne Valley, has joined the Green Party under Zack Polanski. She says two things tipped the balance, and her timing could not be clearer. The Greens now report membership above 78,000, including 10,000 new members in just the past few weeks, with momentum building since Polanski’s win in early September.

Inside Westminster, Polanski says he has held conversations with multiple Labour MPs about crossing the floor, though no names are public yet. If even one goes, Walker’s move starts to look like the starting gun for a mass defection of left-wing Labour figures to the Greens, not a one-off.

For Labour, Walker’s switch is a warning light. For Polanski's Greens, it is vindication that they can become a credible home for disillusioned left-leaning Labour figures and activists, shaped around renters, wages, climate, and an economic agenda rooted in fairness and everday working class struggle, not just a party representing 'none of the above'.

Why Walker matters

[Zack Polanski] is confronting the issues people care about, speaking to the media, and listening to understand the struggles of ordinary people.
- Thelma Walker

Walker’s reasons land in exactly the space Polanski is trying to own. She cites two triggers in her interview with The London Economic. First, Polanski’s leadership style, which she says is focused on the issues people actually live with and on listening. Second, the public meltdown of the Corbyn-Sultana project, which convinced her that neither figure is ready to lead a party.

That makes her a permission slip for others on the Labour left. Once a Labour MP, she later backed cross-party cooperation and even ran as an independent. Joining the Greens is her first formal political home since leaving Parliament. That makes her switch a strong signal that disillusioned Labour figures do have somewhere credible to go.

Potential sitting Labour defections

Speculation about defections is growing in Westminster, with Polanski saying he has held multiple conversations with Labour MPs about crossing the floor. He will “absolutely” welcome defectors who fit Green values. No names are confirmed, so caution should be warranted, but the claim is now on the record in several outlets.

The MPs most often speculated about are those already restless over Starmer’s positioning on Gaza, climate, housing, migration, or any of the numerous issues his Labour government have moved rightwards on.

What the Greens want from defectors

Polanski isn't chasing defectors for headlines alone. The Greens are clear they want MPs who add to the party’s new identity, and continue building on their eco-populist agenda. The pitch is built around three pillars.

First, material politics. The party wants voices who will talk about rent caps, wages, public services and climate in the same breath, rather than treating environmental issues as separate from daily life. Walker’s emphasis on housing and inequality sits squarely in that frame.

Second, credibility with movements. Polanski has courted trade unionists, campus organisers and community campaigners. Any Labour defector who can carry those links into Parliament gives the Greens a direct bridge between grassroots activism and Westminster.

Third, a clean break with Starmerism. The Greens are framing themselves as the only serious alternative to both Starmer’s triangulation and Farage’s reactionary populism. For defectors, that means being prepared to publicly reject Labour’s rightward drift and embrace a more openly left-populist stance.

Polanski has been blunt: defectors shouldn't be solely disaffected with Labour, they need to be aligned with the party’s values. That means no MPs only looking for a lifeboat, but those ready to plant their flag on renters’ rights and climate justice, and who are willing to have a determined focus on a compassionate economic agenda.

What comes next

The next few months will show whether Walker’s decision was an isolated moment or the first sign of a broader realignment. The Greens are targeting record local election gains in 2026, and they know that high-profile defections would give them credibility that no amount of membership growth alone can buy.

For Labour, the risk is obvious. MPs already uneasy with Starmer’s stance on Gaza, housing and the cost of living now have a tangible example of where they might land if they jump. Starmer yesterday recognised a Palestinian state, a move widely read as a signal to those restive MPs. But recognition alone may not ease concerns while Britain continues to supply weapons and intelligence to Israel.

For the Greens, the opportunity is equally clear: convert a protest reputation into a parliamentary bloc rooted in everyday struggles.

Walker’s move is more than one former MP finding a new political home. It is about whether a new gravitational centre is forming on the left of British politics, and whether the Green Party under Zack Polanski can turn momentum into a lasting shift.

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