Tata Steel has brought forward the closure date of Port Talbot to the day before workers were set to strike.
Port Talbot’s steelworks will shut on July 7 instead of in September, with the new closure date being the day before Unite members were set to walk out.
While the first blast furnace at the site was scheduled to close later this month, it is the second furnace’s closure which has been sped up, and is expected to result in 2,800 immediate job losses.
The decision comes as part of Tata's effort to reduce carbon emissions, replacing the old fossil-fuel-powered blast furnaces with a single electric arc furnace.
It aims to support the site's move towards cheaper, greener steel production.
Despite the £500 million in government support for this transition, approximately 2,500 job losses will occur over the following year before a further 300 in the next three years.
Unite General Secretary Sharon Graham said: "Unite is fighting for the future of the steel industry. We have secured serious investment from Labour to safeguard jobs.
“The Unite campaign is not about selling jobs, it's about securing the long-term future of steel making in this country for thousands of workers in Port Talbot and South Wales.”
Labour had urged Tata to avoid closing the site early, asking it to wait till after the election and promising fresh rounds of negotiations should the party be put into power.
A Tata Steel spokesperson said: "Unite's strike announcement was made unilaterally and it unfortunately forced us to commence legal action to challenge the validity of Unite's ballot."
Wales First Minister Vaughan Gething added: "This will cause huge anxiety for the workforce, their families and the community.
“Urgent and good faith negotiations are required to ensure safety on site and avoid a severe and lasting impact on Port Talbot, Wales and the UK."