Ford is to close its engine plant in Bridgend, south Wales, by September 2020, unions have said.
A formal announcement on the future of the plant will be made on Thursday, sparking fears for hundreds of jobs reliant on the plant.
The decision will lead to the loss of 1,700 jobs at the South Wales factory, and many more in companies supplying goods and services to the plant.
The news was being confirmed at a meeting between company officials and union leaders, sources said.
Ford said it does "not comment on speculation" but sources close to the engine plant have told ITV News the news is "devastating".
A Unite spokesperson confirmed its members would meet bosses on Thursday, adding: "Our priority is our members' jobs, the communities and livelihoods in the supply chain that Ford Bridgend supports."
Unite general secretary Len McCluskey described it as "a grotesque act of economic betrayal".
He said: "Ford broke promise after promise to the UK. First, it was that it would build 500,000 engines at Bridgend. That fell to a quarter of a million, then fell again and again to now just 80,000.
"The company has deliberately run down its UK operations so that now not a single Ford vehicle - car or van - is made in the UK.
"Ford has treated its UK workers abysmally, and they could do so because the fact remains that it is cheaper, easier and quicker to sack our workers than those in our competitor countries."
He warned the unions were prepared to fight the plan all the way.
And, Jeff Beck, GMB Organiser, said: "We're hugely shocked by today's announcement, it's a real hammer blow for the Welsh economy and the community in Bridgend.
"Regardless of today's announcement, GMB will continue to work with Ford, our sister unions and the Welsh Government to find a solution to the issue and to mitigate the effects of this devastating news."
Mr Beck had earlier remarked the impending announcement had come as Donald Trump was in the UK "talking up a special relationship and trade deal with the UK and the US"
He said that the new assembly line is likely to be taken to Mexico.
“So much for the special relationship Mr Trump."
Responding to speculation that Ford will close its Bridgend plant, Bethan Sayed, Plaid Cymru AM for South Wales West, said: "If this is true, it will have devastating impact on Bridgend, the region and the wider Welsh economy.
"The Bridgend plant is a hallmark of the Welsh industrial landscape and a flagship anchor company with a highly skilled and specialised workforce. There’s no disguising or minimising the damage a closure will do.
“We must fight this closure and show that the Bridgend plant can be viable. We have to support the workforce in opposing this closure.
"Plaid Cymru is ready to work with others to safeguard the best interests of the workers and to find a way to support them."
Assembly Member for Bridgend, Carwyn Jones, shared his concerns with ITV News and said it would be a "difficult night" for Ford workers awaiting an announcement on Thursday.
- ITV News Correspondent Rupert Evelyn explains how Ford has been running out of road:
Shadow Minister for Business, Russell George AM, added: “The threat to this large and established Welsh employer is very worrying news for the people of south Wales.
“The Welsh Government must move quickly to reassure the automotive business community that Wales is an attractive place to come to set up and stay in. It should be doing more to support Ford during this time.
“I only hope tomorrow doesn’t bring the speculated bad news for workers, after an already bad week for the business community of Wales following the Welsh Government’s rejection of the M4 relief road”.
- Have there been signs of this rumored closure before now?
For more than 40 years Ford have been manufacturing in Bridgend, but in recent years the plant has been running out of business.
Doubts about the future of Bridgend were first raised in September last year when Ford almost halved a planned investment in from £181m to £100m.
At the time the company said the decision had nothing to do with Brexit.
The company then announced in February it was embarking on a programme of "voluntary separation" which would affect hourly and salaried employees based in Bridgend.
Ford announced last month it was cutting 7,000 white collar jobs worldwide, with up to 550 expected in the UK.
The US manufacturer said the cuts represented 10% of its global salaried workforce and 20% of its management positions.
The Jaguar-Land Rover products manufactured at the plant are due to move completely by 2020 and with production of the Sigma engine running down, there is little work left to do.
The Sigma engines made in Bridgend were exported to European markets including Germany and Spain where they are installed in Focus, Mondeo and Fiesta models.
Only the Dragon engine is left but it’s thought the survival of the plant is not viable with that alone.
Figures released by the Office for National Statistics earlier this year showed that the UK's car manufacturing industry is in slow down.