Tata Steel has confirmed plans to close the two blast furnaces at the country’s biggest steel works at Port Talbot in Wales.
Both blast furnaces at the site will be closed and replaced by electric arc furnaces to cut costs and carbon emissions.
Around 2,500 jobs will go at Port Talbot with Tata cutting 2,800 in total across its operations in the UK.
A funding package from the Treasury worth up to £500 million is to be handed over to Tata to pay for the transition with the company injecting £750 million.
Unions have immediately condemned the plan as “unacceptable”.
Roy Rickhuss, general secretary of the steelworkers' union Community said: "The decision to plough ahead with the bad deal for steel first announced in September would be devastating for Port Talbot and the wider steel industry."
Unions had put forward a plan that would have kept one of the blast furnaces open while the transition to electric arc furnaces was underway but this was rejected by Tata.
The Indian group said that the plans are designed to "reverse more than a decade of losses and transition from the legacy blast furnaces to a more sustainable, green steel business.
"The transformation would secure most of Tata Steel UK's existing product capability and maintain the country's self-sufficiency in steelmaking, while also reducing Tata Steel UK's CO2 emissions by five million tonnes per year."
PM Rishi Sunak conceded that the closures and job losses would mean a “worrying time” for everyone affected but that the alternative was for the entire plant to close, which would have meant 8,000 job losses.
Electric arc furnaces use scrap metal as their source material rather than the metallurgical coal, iron ore and hardeners mixed in blast furnaces.