Zack Polanski today urged Britons to vote for his Green Party in order to drag Labour further to the Left on rent controls, wealth taxes, fossil fuels and Gaza.

In his keynote speech to the Greens' conference in Brighton, the self-described 'eco-populist' claimed backing his party was the 'most powerful vote you can make'.

'Vote Green and watch Labour move,' Mr Polanski said as he attempts to stem a flood of Left-wing voters switching back to Labour under new Prime Minister Andy Burnham.

The 43-year-old, who reckons he has a '50-50' shot at beating Labour at next week's Holborn and St Pancras by-election, hailed the impact the Greens have already had on the Government amid their poll surge earlier this year.

'At general elections, the government changes. But between general elections – we change the government,' Mr Polanski said, referring to when Labour MPs ditched Keir Starmer in favour of Mr Burnham.

Just days after Mr Burnham at Labour's own conference delivered what was dubbed the most Left-wing speech by a PM since the 1970s, Mr Polanski unveiled Green proposals for a 'three-year emergency brake' on private rents.

This would see increases for tenants limited by a triple lock-style guarantee.

Mr Polanski also called for his party to keep up the pressure on Labour to retain its ban on new North Sea oil and gas drilling, while also taking a stronger position on Israel and 'deproscribing' Palestine Action.

And he vowed the Greens would 'introduce a windfall tax to obscene bank profits', as he insisted his party would not 'hesitate in taxing extreme wealth'.

But Mr Polanski's attentions appeared mainly focused on the upcoming by-election in Holborn and St Pancras, with the Green leader departing Brighton shortly after his speech to return to campaigning in the north London constituency.

He is not expected to return to his party's gathering on the south coast as he stages a final push for votes this weekend. 

The conference hall was around two-thirds full for Mr Polanski's speech, with Green activists having been encouraged to forgo a stay in Brighton in order to help campaign in Holborn and St Pancras, formerly Sir Keir's constituency. 

After he was elected party leader in September last year, the Greens enjoyed a surge in support and membership numbers under Mr Polanski.

But the party has since seen its poll ratings almost halve since a peak in March, which has coincided with a series of rows and Labour's lurch to the Left under Mr Burnham.

As he opened his conference speech in Brighton, Mr Polanski said his party's 'wild' year had seen it attract 'more members than ever'.

He also hailed the election of 'record numbers' of Green councillors, as well as breakthroughs at the Welsh elections in May and the Gorton and Denton by-election in February.

Mr Polanski said his decision to stand against Labour in Holborn and St Pancras had seen Mr Burnham delay giving the go-ahead to new oil and gas drilling at the Rosebank and Jackdaw fields in the North Sea.

'That makes me hopeful. That is the difference our party makes,' he added. 'Vote Green, and watch Labour move.

'Voting Green, between general elections, is the most powerful vote you can make. And here's two reasons why.

'First – more Greens in elected positions. Making the change our neighbours, our communities, really need.

'Second – at general elections, the government changes. But between general elections – we change the government.'

Mr Polanski claimed it was only because of Green demands that the PM called a COBRA meeting in the wake of recent wildfires across Britain amid repeated heatwaves.

'This summer's fires should have been the final nail in the coffin for new oil and gas,' he said.

'We know that drilling for new fossil fuels won't bring down bills. We know it won't make us more secure. And we know that it locks us into a hotter, more dangerous climate.'

Mr Polanski also suggested it was Green pressure on Labour that had forced Foreign Secretary Ed Miliband to recently announce a ban on the import of goods from illegal Israeli settlements.

'Through our sustained pressure – and our historic wins – we have held the worst instincts of the Labour Government in check and started to move them in the right direction,' the Green leader said.

In further demands on Mr Burnham, Mr Polanski called for the dropping of charges against those who have been arrested for their support of the proscribed Palestine Action group.

'We stand with one voice now as we say: Drop the charges. End the Genocide. Deproscribe Palestine Action,' he added.

Chants of 'Free, free Palestine' could be heard from some Green members in the hall, who were wearing Palestinian keffiyeh scarves around their necks or across their shoulders. 

As he outlined Green plans to limit private rent increases, Mr Polanski called for a triple lock-style guarantee for tenants to ease young adults' housing costs.

He set out proposals for a 'three-year emergency brake', which would temporarily limit rent rises to the lowest of either Consumer Prices Index (CPI) inflation, wage growth or 2 per cent.

These are similar metrics to the ones used to uprate the state pension each year, which rises by the highest of either inflation, wage growth or 2.5 per cent.

'For too long, landlords have been able to hike prices way beyond wages,' he said. 

'Leaving renters forced to meet the cost to keep a roof over their heads. Cutting back and back and back on everything else.'

Mr Polanski claimed the plan would also save the taxpayer money through lower spending on housing benefits used to subsidise 'extortionate rents'. 

'No rent rises above CPI, wage growth or 2 per cent – whichever is lower,' he added. 'No sudden hikes. No evictions under the guise of rent rises. No more uncertainty, no more fear.

'That emergency brake would give renters vital breathing space. Let wages begin to catch up with the rents that have outstripped them for far too long.

'And in the long term, we'll put in place a national Fair Rents Guarantee – to keep rents affordable for everyone, for ever.'

As he ended his conference speech, Mr Polanski said the Greens' 'differences with this Labour Government could not be clearer' and they are 'just getting started', following his plea for voters to back his party in order to influence Mr Burnham's administration.

Party officials admitted there had been a 'change in emphasis' in Mr Polanski's message to voters from his declaration last year that the Greens were bidding to 'replace' Labour under Sir Keir's leadership. 

'It's a change of emphasis rather than the fact that we think we can't replace the Labour Party,' a Green spokesman said.

Pressed on whether Mr Polanski would take the Greens into a coalition with Labour after the next general election, they said it would be 'up to members'. 

They also suggested Labour had timed the Holborn and St Pancras by-election in order to disrupt the Greens' conference.

'The timing of the by-election was not of our choosing,' the spokesman added when asked about Mr Polanski's fleeting appearance in Brighton.

'It was of the Labour Government's choosing, and I suspect they very well knew what they were doing when they chose next Thursday as the day.'

Mr Polanski earlier insisted he won't be forced to quit as Green leader if he fails to win the Holborn and St Pancras by-election.

He described the contest as being 'on a knife edge' and 'probably 50/50 right now' as he seeks to take the constituency from Labour.

But activists were being encouraged to ditch the party's conference in Brighton this weekend to go and join the by-election campaign.

A banner adorning one empty conference stall told party members: 'Stop Reading This. Go campaign in Holborn & St Pancras to elect our 6th Green MP.'

It follows reports that Mr Polanski failed with an attempt to postpone the gathering entirely in order to focus the party's efforts on his bid to join the House of Commons.

Mr Polanski has gambled on standing in Holborn and St Pancras despite the Greens' plummeting opinion poll ratings and his growing unpopularity with voters.

He is widely expected to lose to Labour in the contest, which was sparked by Sir Keir quitting Parliament following his ousting as prime minister.

The start of the Greens' conference in Brighton this weekend has been accompanied by rumours of a potential leadership challenge from one of Mr Polanski's deputies.

Mothin Ali, a Leeds City councillor, is reported to be 'gearing up' to run in the next Green leadership election due next year.

There was also a dispute over whether TV channels could air Mr Polanski's keynote speech live on Friday afternoon, due to strict by-election rules.

Meanwhile, the Brighton gathering is set to be dominated by a furious row over a planned debate by Green members on a motion to declare Zionism as a form of racism.

Caroline Lucas, the former Green leader, has branded the motion 'divisive' and warned it 'could be seen as a sign that Jewish people aren't welcome in the party'. 

Speaking to The Guardian ahead of his keynote conference speech on Friday, Mr Polanski denied his leadership could come under threat if he failed in Holborn and St Pancras.

'It doesn't depend on it whatsoever,' he told the newspaper, adding that the by-election had ramifications for politics nationally but 'not for my leadership'.

'I'll be completely honest, it won't surprise you to know that I want to win the seat,' Mr Polanski said.

'I think the seat is winnable, and I think it's probably 50-50 right now. It's really, really on a knife edge.'

Speaking at her party's own conference in Liverpool this week, Labour deputy leader Lucy Powell said she is 'confident' of winning the Holborn and St Pancras contest.

She claimed many Labour voters who left the party for the Greens during Sir Keir's premiership were giving the party 'a second look' now Mr Burnham is in power.

A defeat for Mr Polanski in next Thursday's by-election would represent a huge blow to his leadership.

Rob Ford, professor of political science at the University of Manchester, said Mr Polanski's first year as Green leader had 'brought many more voters off the fence, but most have broken against him'.

'This big negative shift has coincided with Labour installing a more popular leader and with a large positive shift in Conservative leader Kemi Badenoch's ratings,' he wrote in an article for Sky News.

'Polanski and the Greens need to decide how to respond to Labour's moves to close down space on the Left.'

He added: 'Regardless of which way they turn, the Greens may also need to ask themselves whether Polanski remains a political asset after a turbulent first year as leader, which has seen his poll ratings fade fast.

'Though Labour gains from the Greens are much more evident since Andy Burnham became PM, Green polling had started to decline even before his arrival.'

However, Rachel Millward – another Green deputy leader – claimed her party and Mr Polanski had 'set the political agenda' in the past 12 months. 

She told Times Radio: 'The reality is that it's Zack Polanski who brought the nation Andy Burnham.

'And if we hadn't had the success of the Green Party in this last year since Zack's been leading, which has been unprecedented, phenomenal success...

'...if we hadn't had that much pressure on Labour, then we'd probably still be in the Starmer regime, which was pretty much ushering in a Reform government with its lurch to the Right.

'So actually, what we're seeing now is that we've set the political agenda and the Government is picking that up.'