Green Party leader Zack Polanski has committed £8.4billion to shield British households from rising energy costs, announcing the policy on Wednesday.
Speaking at a community garden in north London, Mr Polanski urged ministers to guarantee that bills would not exceed the April to June price cap level.
The intervention would be funded by aligning capital gains tax rates with income tax, a measure the Greens estimate would generate £12billion annually for the Treasury.
"The alternative is unacceptable: if the price cap rises, we will see interest rate rises, mortgage rates up, bond yields up, and inflation up and we will be back into the doom loop that has done untold damage to our economy," Mr Polanski said.
The former theatre performer acknowledged the proposal was "not cheap" but said action was necessary.
Mr Polanski criticised the Chancellor’s approach to public finances, describing Labour’s fiscal framework as a "black box" that Rachel Reeves adheres to "no matter what, even when we can see it's crushing the economy, eroding workers' rights and destroying our public services".
He argued current rules requiring debt to fall as a proportion of GDP by the end of the forecast period are flawed.
"Our fiscal framework is hypersensitive to market movements and this creates policy uncertainty that then fuels the very market jitters it is there to supposedly prevent," Mr Polanski said.
The Green leader unveiled a £8.4billion plan to cap energy bills
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Mr Polanski proposed establishing a panel of independent experts to assess debt sustainability and appropriate spending levels, drawing on models already operating in Australia and New Zealand.
He also questioned the use of GDP as a measure of economic success.
"GDP is a terrible way to measure the health, happiness and wellbeing of a country," Mr Polanski said.
He added he is "much more interested in growing people's mental health, growing our public services, growing cohesion of our communities".
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He was highly critical of the Chancellor
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On housing, Mr Polanski pledged nationwide rent controls across England to tackle what he described as "rip-off rents".
"If we had frozen rents in autumn 2022, households in Britain would be saving over £3,300 per year on average," he said.
The address comes as the Greens continue to poll strongly, with the party currently sitting fourth in national polls at around 16 per cent support.
Last month, the party won the traditionally Labour-held seat of Gorton and Denton in a by-election.
Rising global tensions have contributed to higher gas prices
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Rising global tensions have contributed to higher gas prices, with analysts at Cornwall Insight forecasting the household price cap will climb to £1,827 for July to September, up from £1,641 in the current quarter.
Mr Polanski said families felt "helpless" amid global uncertainty and called on the Government to "insulate us from some of the worst economic effects of Trump's war".
The Greens’ £8.4billion estimate assumes prices rising by up to £300 per household once Ofgem recalculates the cap.
Other parties have also set out alternative approaches, with Reform UK proposing to scrap VAT on energy bills entirely.
The Government has so far announced £53million in targeted support for vulnerable households affected by heating oil price rises.
Ms Reeves has indicated any broader assistance would focus on the poorest households rather than universal support.
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