Tata Steel has told workers it could to cease operations at its steel plant in Port Talbot months earlier than planned because of a strike.

The company had been planning to shut down one of the blast furnaces by the end of June and the second one by September. But workers at the south Wales site have been told that Tata plans to cease operations at both furnaces no later than 7 July because of the strike by members of Unite, which starts the following day.

The company said the strike meant it could no longer be assured of sufficient resources being available to ensure safe and stable operations. A Tata Steel spokesperson said: “In the coming days, if we cannot be certain that we are able to continue to safely operate our assets in a stable fashion through the period of strike action, we will not have any choice but to pause or stop heavy end operations (including both blast furnaces) on the Port Talbot site.

“That is not a decision we would take lightly, and we recognise that it would prove extremely costly and disruptive throughout the supply chain, but the safety of people on or around our sites will always take priority over everything else.”

Unite called the strike in protest at plans to switch to a more environmentally friendly way to produce steel, with the loss of thousands of jobs. The union’s 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. Tata putting out a statement to shut or pause its blast furnaces three months earlier than they intended to is the latest in a long line of threats that won’t deter us.

“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.

“We call on the real decision makers in Mumbai to take hold of this dispute, sit down, negotiate and realise that the investment secured will be good for the company and workers.”

Charlotte Brumpton-Childs, the GMB national officer, said: “This is a sad day for steel. Tata must step back from this irreversible decision and safeguard steelmaking assets. There’s a general election in days that could change everything so much.

“We know there is a future for steelmaking in south Wales. That future must be preserved. GMB’s next actions will be decided by our members.”

The Community and the GMB unions are also campaigning against the plans but have ruled out taking industrial action before the general election. The unions expect an incoming Labour government to hold emergency talks with Tata to discuss alternatives to its proposals.

Tata has made it clear it is pressing ahead with switching from blast furnace production to a greener electric arc furnace and it is investing £1.25bn into the project. The company says it is currently losing £1m a day at Port Talbot, which it has warned is unsustainable.

The company offered an enhanced redundancy package to workers affected by the plans, but this was reduced after Unite members started industrial action short of a strike earlier this month.