Workers at steel giant Tata are to ban overtime as part of industrial action in protest at job losses.
Unite said around 1,500 of its members based in Port Talbot and Llanwern in South Wales will take action from June 18.
Unions are opposed to the company’s plans to close blast furnaces and switch to a greener form of production which needs fewer workers.
Unite warned that strike action will be held if the company does not row back on its plans.
Unite general secretary Sharon Graham said: “Tata has miscalculated.
“Its workers know a better future is possible and they will be taking industrial action to fight for it, with Unite using every tool at its disposal to make the company change course.
“This hugely wealthy company knows UK steel capacity and jobs can be retained as the transition is made to green steel.
“Tata’s disastrous deal with the current Government would only see its other overseas operations take advantage of the coming boom in green steel at the expense of South Wales.”
Other unions are campaigning to save jobs at Port Talbot but have not announced any industrial action.
Tata says it has no option but to change the way steel is produced, adding that it has made generous redundancy offers to workers affected by its plans.
A Tata Steel spokesperson said: “We are naturally disappointed that Unite has decided to ask its members to take industrial action at Port Talbot and Llanwern.
“We have written to Unite twice during the ballot process, and again this week, to notify them of significant irregularities in the ballot process they have undertaken. We will continue to review and consider our legal next steps.
“On 22 March, we put forward a significantly enhanced, comprehensive package of support for employees impacted by the proposed transformation.
“Following further discussions with the multi-union UK Steel Committee, the company has put forward a further enhanced offer in good faith and in order to finalise discussions on the proposed Memorandum of Understanding covering our restructuring and transformation, forward aspirations for the business, and the employment support package arrangements.”