Ford's engine plant in Bridgend is set to close in autumn 2020, with the loss of 1,700 jobs.
Union officials were told of the plans at a meeting with Ford bosses which include the offer of redeployment.
Workers were sent home after receiving a letter which said they will lose their jobs in phases by 25 September next year.
Ford blamed "changing customer demand and cost" for the closure plans and denied Brexit was a factor.
In a statement, Ford of Europe president Stuart Rowley said creating a sustainable business required the company to make "difficult decisions", including the need to make its engine manufacturing base suitable for the vehicles it produces in the future.
"We are committed to the UK. However, changing customer demand and cost disadvantages, plus an absence of additional engine models for Bridgend going forward make the plant economically unsustainable in the years ahead."
Later, he said the decision was nothing to do with Brexit although he realised the company's plans would be "very significant for the employees, their families and the community in south Wales".
Mr Rowley has confirmed the company will repay £11m in incentives offered by the Welsh Government.
Workers at the plant said they were devastated.
"I'm expecting to lose my job," said Tony Phillips, who has been at the plant for 31 years, adding that they were "good, well-paid jobs".
Fellow worker Mark Lendrum said: "South Wales is going to be like a ghost town."
Leader of Bridgend council Huw David, said there is "not a family in Bridgend that won't be affected by this" and it is a "fabulous workforce" from Newport to Llanelli.
"We don't know how we are going to recover from this.
"Both governments want to keep the jobs here. We as the council will help people to find other work," he said.
The closure news comes just months after Ford said it was cutting its Welsh workforce by 1,000, with 370 going in a first phase.
Investment in a new petrol engine, called Dragon, was scaled back, while production of an engine for Jaguar Land Rover is due to end this year.
Just days ago, it was revealed that car sales in the UK had fallen again.
GMB regional organiser Jeff Beck said: "We're hugely shocked by today's announcement. It's a real hammer blow for the Welsh economy and the community in Bridgend."
He said the union would continue to work with Ford, other unions and the Welsh Government "to find a solution to the issue and mitigate the effects of this devastating news".
Unite union general secretary Len McCluskey said Ford had treated its UK workers "abysmally".
"The fact remains that it is cheaper, easier and quicker to sack our workers than those in our competitor countries," he said, vowing to "resist this closure with all our might".