Around 1,500 workers at Tata Steel will hold an “all-out indefinite strike” next month, a union has announced.
The industrial action at the company’s sites in Port Talbot and Llanwern, Newport, will begin on 8 July, Unite said.
The union said the walkout would “severely impact” the company’s UK operations.
It comes in response to plans to close Tata Steel’s blast furnaces in South Wales, putting 2,800 jobs at risk.
The union said it would be the first time in more than 40 years that steel workers in the UK have gone on strike.
Members voted in favour of the move in April.
Industrial action short of a walkout, including staff working to rule and a ban on overtime, began earlier this week.
The union’s general secretary Sharon Graham said: “Tata’s workers are not just fighting for their jobs – they are fighting for the future of their communities and the future of steel in Wales.
“Our members will not stand by while this immensely wealthy conglomerate tries to throw Port Talbot and Llanwern on the scrap heap so it can boost its operations abroad.”
She added: “The strikes will go on until Tata halts its disastrous plans.
“Unite is backing Tata’s workers to the hilt in their historic battle to save the Welsh steel industry and give it the bright future it deserves.”
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Tata Steel previously said it was losing £1m a day at Port Talbot and warned the situation was unsustainable.
The company said its plans, which include the building of an electric arc furnace, would mark the beginning of a new way of “competitive and greener” steelmaking.
The proposals were officially confirmed in January, with its boss TV Narendran telling MPs the decision was “pretty much” a done deal.
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Sky News first exclusively revealed details of the plans in September 2023.
Tata Steel initially offered an enhanced redundancy package to workers affected by the proposals, but this was reduced after the industrial action short of a strike began earlier this week.
Unions, including Unite, expect Labour to hold emergency talks with the company if the party wins the upcoming general election.
Alun Davies, national officer for steel at the Community union, which says it represents the “vast majority” of affected Tata workers, said it had decided with the GMB union not to take part in any industrial action for now.
He added: “If the Labour Party wins the general election, it has said that it will hold emergency talks with Tata.
“We welcome this, and now feel it is important to wait for the completion of that process before initiating any significant course of action.”