Backsliding on climate action would drive the Labour party into political obscurity, Zack Polanski has warned, as trade union leaders said more drilling in the North Sea would not help UK workers.

The Green party leader, speaking to the Guardian as searing heat swept the country for the second time this year, urged Andy Burnham – widely expected to be the UK’s next prime minister – to be bold on climate justice. He said any move to water down the party’s commitments would have dire consequences at the ballot box.

“Half measures or backsliding on climate action would be a moral and political failure from Andy Burnham. He has the chance to be bold, and failing to do so will see our country get poorer and his party slip further into obscurity.”

The leader of the UK’s biggest union, Unison, has called for no more drilling for oil and gas in the North Sea amid a debate within Labour and the trade union movement about the party’s climate commitments.

However, the former health secretary Wes Streeting has called for more drilling in the North Sea, including giving the go ahead to the massive Rosebank oilfield. Sharon Graham, the leader of Unite, which represents workers in the oil and gas industry, also backs new drilling and said that the commitment of the energy secretary, Ed Miliband, to net zero would be a “noose around the neck” of job creation. The British Chambers of Commerce said Burnham should exploit what fossil fuels were left in the dwindling North Sea basin “to avoid mass job losses”.

Critics point out that the number of jobs supported by the industry has more than halved in the last decade – from 441,000 to 214,000 – despite previous governments issuing hundreds of new licences. Statistics show between 90% and 93% of all viable oil and gas has already been drained from the basin.

Yet in debates over who should be Burnham’s chancellor, some on the right of the party and the union movement back Streeting over Miliband.

A senior trade union source said there was widespread unease within the union movement at Unite’s pro-drilling stance. “Many in our union – and other unions – are worried that Sharon’s interventions are boosting [Nigel] Farage and his crypto-backers,” they said. “And that her attack on Ed played right into the hands of the Labour right. Wes as chancellor would be a bad outcome for the working class, including Sharon’s members. It’s not strategic at all.”

Polanski said any move to boost fossil fuel production would seal Labour’s fate. “Decades of inaction on the transition to clean energy has already put the UK way behind where we should be, with huge cost to the economy,” he said. “As the country swelters under extreme heat there has never been a more prescient reminder that we simply cannot afford to keep burning fossil fuels.”

A recent CBI report found the net zero economy was worth about £100bn a year to the UK, growing faster than the rest of the economy and producing higher-paid jobs.

Other unions also say a Burnham government must double down on its climate plans to secure a fair and sustainable future. Steve Wright, the general secretary of the Fire Brigades Union, said its members were on the frontline of the climate crisis dealing with wildfires and heatwaves. He added: “We see first-hand the need for urgent climate action and that must include restrictions on drilling in the North Sea.

“We urgently need a plan for adaptation that protects people’s health as well as the environment. Otherwise, firefighters will still be pushed beyond limits to respond to the sheer scale of wildfires this summer and in future years.”

Andrea Egan, the leader of Unison, said this month that more drilling for fossil fuels would do nothing to help working-class people.

“Climate change denial is creeping into politics like never before, with far-right parties treating fossil fuels as a panacea for the country’s problems,” Egan wrote. “Plundering the North Sea wouldn’t make a significant difference for working-class people in Britain, and it would be grossly irresponsible to working-class people in the global south.”

The UK’s largest teachers’ union, the National Education Union, also urged Burnham not to back away from climate action. “This week’s extreme heatwave is causing untold disruption and discomfort throughout our education system,” said Daniel Kebede, the NEU’s general secretary. “This certainly isn’t the moment to move away from trying to tackle the very causes of what we are experiencing this week; if we make that mistake, these episodes will only continue to get worse.”