More than 2,800 jobs will be axed as Tata Steel plans to close its blast furnaces at the Port Talbot plant in South Wales in the UK.
Of the total job cuts, 2,500 will be in the next 18 months, said Indian conglomerate Tata. A further 300 jobs are to go in three years.
Statutory consultation on the cuts will begin, Tata Steel said but did not specify when.
Voluntary redundancies will be sought and more than 130 million pounds will be committed to a support package for affected employees.
It comes as Tata Steel announced it was to press ahead with plans to close blast furnaces at the Port Talbot plant in South Wales.
The first blast furnace will be closing around mid-2024 and the remaining heavy end assets will wind down during the second half of the year, Tata Steel said on Friday.
The steel giant in September said that it had secured 500 million pounds of taxpayer cash to support the site's transition to cheaper, greener steel production to cut emissions and stem financial losses.
Job cuts had been expected as part of the deal though 5,000 UK staff are due to remain within the wider UK operations following the agreement with the government.
The redundancies, to be completed by March next year, will see three-quarters of the 4,000 staff on site put out of work.
The Port Talbot workforce currently accounts for 12 per cent of the coastal town's entire population and many had expressed concerns for their families' futures when it emerged that big job losses were expected.
Stephen Kinnock, Labour MP for Aberavon, home of the Port Talbot plant, told Sky News: "Steel is the beating heart of manufacturing and of our entire infrastructure and, of course, of our national security.
"Do we really want to be a country, given the dangerous and turbulent world in which we live, that isn't able to produce its own steel?
"We are very proud of what the Port Talbot steelworks means for Wales and for the entire United Kingdom and we feel passionately here.
"There isn't a single household in my Aberavon constituency that isn't connected to the steelworks in some way, and the impact would be utterly devastating."
Unions met the company on Thursday after presenting alternative proposals aimed at saving jobs but sources said Tata rejected them.
The India-based company will replace the two blast furnaces at the plant with electric arc furnaces under the plan to reduce emissions and costs.
With the closure of both blast furnaces at the Port Talbot plant, related facilities such as coking ovens and the steel shop would also be shut down.
Tata Steel said the plant will continue to roll slab into hot-rolled coil (HRC) for a transition period, importing from Tata India, the Netherlands and other "strategic suppliers" on the international market.
The company has already been importing HRC, amid stability problems with its furnaces and the hot strip mill.
"Around 2,500 jobs will be affected over the next 18 months," the company said. The continuous annealing processing line (CAPL) will close in March 2025.
Continued HRC imports will likely mean that changes will need to be made to import quotas under the UK's steel safeguard, as historical volumes do not account for Tata importing so much material. Its recent imports have strained the 'other countries' quota for HRC, while the Indian hot-dip galvanised quota has also filled quickly in the past two quarters.
The Community Trade Union said Tata and the UK government "must reconsider their positions in order to safeguard the future of British steelmaking, and head off a major industrial dispute".
The government has agreed to give Tata 500 million pounds towards the installation of a new 3 million tonnes per year electric arc furnace, which will be operational by 2027. As part of the deal, Tata agreed to invest 700 million pounds.
But unions have lambasted the deal, which they say has ignored other options such as direct-reduced iron, which could allow Tata to continue producing virgin iron in the UK.