Keir Starmer has accused the Tories of being "asleep at the wheel" over the fate of the steel industry as he vowed to save the sector from a cycle of "crisis and bailout".
The Labour leader paid a visit to the vast British Steel site at Scunthorpe to discuss plans to pour £3billion into slashing emissions from the sector.
Mr Starmer told the Mirror that a good, secure steel industry is a “massive prize” for jobs and the wider economy - and it would be “unforgivable for this Government to simply sit it out”.
Shadow Climate Change Secretary Ed Miliband said the embattled industry was facing a "critical moment" amid soaring energy costs and competition from cheaper imports.
"Is the Government basically going to wash its hands of our steel industry? I think that's what the Tories are doing," he said.
"When I speak to people across the steel industry, there's just a sense now that they are waiting for an election for actually something to happen because they are not going to get the support from the Government."
It comes as part of Labour's broader green energy blueprint, which is expected to include a block on new North Sea oil and gas drilling.
Mr Starmer is expected to unveil details of his plan to make Britain a clean energy super power later this month.
He believes Tory failings have left the UK energy market exposed to international pressures like the war in Ukraine.
“(Vladimir) Putin has got his boot on our throats and that is because the PM and the Tories have failed for 13 years to put us in a position where we weren’t exposed," he said.
The Labour leader added: “This isn’t about politicians trading words, it’s families paying higher bills, energy intensive businesses paying higher bills.”
And he insisted “nobody is talking about turning off the tap” on oil and gas drilling.
Labour wants to shift towards renewable energy, and argues that construction of offshore wind, solar and nuclear power projects will drum up demand for British-produced steel.
The party wants to pour cash into the sector from a proposed national wealth fund, which will back investment in new technology.
Mr Starmer said: "The demand for steel is only going to go up. And we want that to be British steel."
The current model is "coming to the end of its life", leaving the struggling industry reliant on a cycle of bailouts and takeovers, he said.
Decarbonising the sector provides an opportunity to provide cleaner, cheaper steel, Labour says, - and allows for a secure supply of British-made material for defence contracts.
Mr Starmer said: "That's a massive opportunity and an incoming Labour Government would seize that opportunity because there's a chance to get ahead here."
Steel workers told the Labour leader there was demand for low carbon steel but that demand can't be met at present.
He said: "There is a very strong sense that the Government is asleep at the wheel.
"It is obvious we need this transition and we need a Government that wakes up to that and has a plan in place to seize that opportunity - and is prepared to do the hard yards and take the tough decisions...This is a firm plan, not just to save our steel, but also to give our steel the future that it deserves."
The £2.4billion steel industry employs 33,700 people directly in the UK and supports a further 43,000 jobs in supply chains.
The Mirror has been campaigning to Save our Steel since 2015.
Roy Rickhuss, general secretary of the Community union, which represents steelworkers, said: "Labour's plans for an active industrial strategy and a £3billion green steel, investment fund can be the game-changers our industry so desperately needs."
A Government spokesman said: "Steel plays a vital role in the UK economy, supporting local jobs and economic growth.
"We want to secure a decarbonised, sustainable and competitive future for the UK steel sector, and we are working closely with industry to achieve this.
"We have made an offer of support to British Steel, and continue to engage actively with the company to ensure any investment would be successful and make effective use of taxpayers' money."
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