Ford wants to cut 370 workers at an engine plant in south Wales in the first phase of almost 1,000 job losses, BBC Wales understands.
Unions have pledged to fight compulsory redundancies at the car giant's plant in Bridgend after they were briefed by Ford management on Friday.
It is believed the first tranche of cuts would be offered as voluntary redundancies.
Ford is looking to shake up its European operations.
Bridgend makes engines for Jaguar Land Rover (JLR) but that contract finishes at the end of 2019, at around the same time the plant will stop making the Ford Ecoboost engine
The factory, which employs about 1,700 workers, won the investment for Ford's latest petrol engine - the Dragon - but that will only employ around 500.
If plans go ahead, the 990 jobs to be lost at Bridgend - almost half of the site's workforce - will go in two phases by 2021, as part of 1,150 losses across the UK.
Ford declined to confirm the figures and said it was currently consulting with unions.
It said these talks were ahead of it implementing a "comprehensive transformation strategy".
Jeff Beck, GMB organiser, said the union would "fight for every Ford job" in Bridgend and across the UK.
"We have been asking the company for two years to clarify the situation regarding jobs and it's not until today that we have had the devastating answer.
"We have now been told 990 jobs will be cut in Bridgend by 2020. This is devastating news for the dedicated workers at Ford and their families.
"Our members there have been extremely loyal to Ford, and we will stand by them."
The Unite union called it "grim news" and said shop stewards had been given a briefing.
"It is a devastating blow for our members and their families, as well as having grave implications for the Welsh economy and the supply chain," said officer Des Quinn.
"Unite is fully committed to opposing any compulsory redundancies and campaigning strongly for Bridgend to have a viable future."
He said representatives would consult with members over the coming days.
"There are a number of factors behind this grim news - the main ones being challenging market conditions for carmakers generally, a lack of a coherent industrial strategy from the UK government and the uncertainty created by Brexit.
"Over the last two decades the UK car industry has experienced a renaissance of which we can all be proud of.
"The challenge for government, the carmakers and the unions in the near future is to fight very hard to maintain the environment that made that success possible."
Analysis by Sarah Dickins, BBC Wales economics correspondent
We have already heard this week about Ford's review of its European operations - as well as the cutbacks at Jaguar Land Rover.
But this news today is something that unions have feared for nearly two years.
This is because it was widely known that Bridgend's contract to make engines for JLR was coming to an end and that Ford's own plans for a new engine had been scaled right back.
Ford have said discussions have only just started and details of numbers are "premature".
But we understand that early talks are around a first tranche of job losses involving voluntary redundancies.
Carwyn Jones, Labour Bridgend AM and former first minister, said he and Labour colleague Huw Irranca-Davies would be working to ensure the plant's future.
"It does reflect the uncertainty that all Ford workers face," he said.
"I spoke to Ford yesterday. They gave no indication of any particular threat to Bridgend or any other plant for that matter, but they did say they were rethinking the way Ford were operating in Europe," he added.
Plaid Cymru's Rhun ap Iorwerth called for an urgent economic summit to be organised.
"These reports are devastating - both for the workers directly affected and the wider Welsh economy," he said.
The 1,000 losses would come alongside 150 in Ford's transport operations, which would affect lorry drivers.
On Thursday, the company said it would be speeding up plans to cut structural costs and thousands of jobs would go across Europe.
"We are taking decisive action to transform the Ford business in Europe," said group vice president for Europe, Steven Armstrong.
"We will invest in the vehicles, services, segments and markets that best support a long-term sustainably profitable business, creating value for all our stakeholders and delivering emotive vehicles to our customers."
The announcement from Ford came on the same day as Jaguar Land Rover said it was axing 4,500 jobs and Honda said it was halting production for six days after Brexit.