Tata Steel has confirmed plans to close blast furnaces at its plant in Port Talbot in South Wales, with the loss of more than 3,000 jobs.
About 2,800 jobs will go over the next 18 months, with a further 300 to be lost later.
The closures are part of India-owned Tata Steel's plan to turnaround its loss-making UK steelmaking business by switching to lower carbon electric arc furnaces, a proposal backed by £500m of government money.
The electric arc furnaces are operated by fewer workers compared to the blast furnaces.
Tata Steel employs more than 8,000 people in the UK.
"Tata Steel today announced it will commence statutory consultation as part of its plan to transform and restructure its UK business," the company said in a statement.
"This plan is intended to reverse more than a decade of losses and transition from the legacy blast furnaces to a more sustainable, green steel business," it stated.
"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 and overall UK country emissions by about 1.5%," the company added.
