Hundreds of steel workers are set to go on strike over the proposed closure of Port Talbot's blast furnaces.
Unite the Union announced the result of its strike ballot on Thursday but walkout dates are yet to be confirmed.
Around 1,500 workers based in Port Talbot and Newport voted in favour of strikes.
It will be the first time in more than 40 years that Port Talbot steel workers have gone on strike.
Thousands of jobs could be lost in the South Wales town should Tata Steel proceed with proposals to shut both blast furnaces.
Unions are warning that a total of up to 2,800 jobs could be lost if Tata's plans go ahead.
Unite's general secretary Sharon Graham described the ballot result as a "historic vote".
"Unite will be at the forefront of the fight to save steelmaking in Wales," she added.
"We will support steel by all and every means."
'Competitive and greener'
The proposals were officially confirmed in January, with Tata boss TV Narendran telling MPs the decision was "pretty much" a done deal.
Tata said the move will mark the beginning of a new way of steelmaking which is "competitive and greener".
But there are concerns in Port Talbot about its impact on the local economy, with the plant's workforce currently accounting for 12% of the town's population.
Unite argues that other European countries are transitioning to ensure a "bright future" for the steel industry.
It says the blast furnaces at the Tata plant in the Netherlands is being kept open and that in Germany more steel is being produced at a single plant than by the entire UK industry.
Unite Wales regional secretary Peter Hughes said its members would "not be intimidated into standing by".
"Our members have their union's absolute support in striking to stop these cuts - Unite is backing them every step of the way," he added.
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Tap hereA Tata Steel spokesperson said the company was "naturally disappointed" by the decision.
The spokesperson added that Tata was engaged in a consultation process with the unions on its proposals.
They said the consultation "continues in an open, collaborative and constructive fashion".
The firm says it has written twice to Unite during the ballot process to "notify them of significant irregularities in the ballot process they have undertaken".
"While the £1.25bn commitment with the UK government will ensure a long-term viable future for low-CO2 steelmaking in the UK, our current business is unsustainable, reporting losses of more than £1m a day," the spokesperson added.
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