Unite has said up to 1,160 jobs will go by 2021 after Ford said it will cut investment in a new engine programme.
The union has warned workers at Ford's Bridgend plant in Wales will be "one step closer to strike action" if the company doesn't commit to protecting jobs. At the moment, the headcount is 1,760.
Ford released an updated statement, saying it had shared a five-year outlook with its union partners, based on what the firm knows today.
"This shows healthy volumes to occupy the current workforce over the next 2-3 years," it said.
Beyond that, identified workload is reduced and whilst such a forecast is not unusual, given the cyclical nature of our business, it is a concern, and we fully understand that.
Ford has therefore proposed a joint-working party with its union partners, Unite and GMB, to identify future business opportunities.
It goes without saying, that in order to attract new business, the Bridgend operation would need to ensure its competitiveness, and addressing some of the current concerns relating to the plant's efficiency would be high on the agenda.
Unite general secretary Len McCluskey is visiting the site today with three mass meetings being held.
He said: "Unite members are committed to Ford, the Welsh Assembly is committed to Ford, all we need now is for Ford to commit to its workforce. That means investment, new product lines and a clean plan for the plant's future."
Meanwhile, GMB organiser Jeff Beck said: "The nightmare for our members at Bridgend has unfortunately come true." He said the union's first priority is to "defend our members' jobs" and will consult with them before deciding what action to take.
The Bridgend plant makes around 250,000 engines a year for Jaguar Land Rover, as well as 500,000 of Ford's Sigma engines, but it is due to stop producing both.
A decision to cut investment in a new Dragon engine plant was announced back in September, with the car firm saying it would halve production of its new generation of petrol engines in south Wales.
At the time the company said the decision had nothing to do with Brexit, though the company said in January, weakening sterling had impacted the firm and could cost it as much as $600m (£478m).
Leaked documents seen by BBC Wales projected the aforementioned reduction, leaving 600 workers at the site. The forecast is made based on the assumption the plant wouldn't be able to bring any new work in.
A spokesman for the First Minister said they were "actively seeking urgent assurances" from Ford, both in the UK and the US.