Steel giant Tata is set to close its Orb Electrical Steels plant in Newport with the loss of nearly 400 jobs.
It announced the decision after a sales process for the plant failed to find a buyer.
Tata put the specialist plant, a manufacturer and processor of electrical steels and part of its Cogent Electricals Steels arm, up for sale last year.
Indian-owned Tata, which recently abandoned a proposed merger with German conglomerate ThyssenKrupp, said the plant had been loss-making for sometime and that converting into an operation for future electric vehicle production would cost in excess of £50m in what was already a very competitive marketplace.
The plant currently directly employs 380, but supports many more jobs in its wider supply chain.
Steel trade union Community, described it as "shocking news, and "made a mockery" of the understanding it had reached with Tata around the jobs guarantee.
Chief executive of Tata Steel's European operations, Henrik Adam, said: "Continuing to fund substantial losses at Orb Electrical Steels is not sustainable at a time when the European steel industry is facing considerable challenges.
"We saw no prospects of returning the Orb business to profitability in the coming years.
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“I recognise how difficult this news will be for all those affected and we will work very hard to support them.”
Tata also announced plans to close its Wolverhampton-based Engineering Steels business, which includes a sales office in Bolton, potentially affecting up to 26 jobs.
Tata has though confirmed a sale and purchase agreement for Cogent Power Inc (CPI), with Japanese steel giant JFE Shoji Trade Corporation.
CPI manufactures cores for electrical distribution transformers and employs nearly 300 people.
Furthermore Tata Steel has decided to retain Surahammars Bruks AB, which makes advanced steels for electric vehicles and employs around 100 people.
On the plant closures in Newport and Wolverhampton, general secretary of the steelworkers' trade union Community, Roy Rickhuss, said: "This is shocking news, which makes a mockery of the understanding we reached with Tata around the jobs guarantee.
"There has been no consultation about this proposal either at 海角视频 or European level and company management should hang their heads in shame in the way this has come about. This is of course extremely devastating news for the workers affected, but all Tata Steel workers should be concerned by the way Tata is breaking its commitments.
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“We are opposed to plant closures and compulsory redundancies and therefore the company should be aware that we will be considering all options, up to and including industrial action, in defence of our members’ livelihoods.
“We also call on the government to intervene. Ministers were closely involved in securing commitments from Tata over the future of the business. At a time when the government wants to decarbonise the economy and support a switch to electric vehicles, losing the 海角视频’s only producer of electrical steels would be a grave error.”