Brexit is "playing a role" in plans by Ford to cut more than 1,000 jobs at its Bridgend plant, according to Unite head Len McCluskey.
It is believed the car manufacturer aims to slash the number of jobs in South Wales from 1,760 to around 600 by 2021.
Speaking to ITV News, Mr McCluskey described the plan as "worrying times" for the region, and admitted Britain's decision to leave the European Union was likely to play a part.
But Mr McCluskey, who is running for a third term as General Secretary of the union, maintained he would be fighting to secure a future for the Bridgend plant.
ITV News believes that a fall in global demand and problems with efficiency - including restrictive work practices - specific to Bridgend, have brought about the proposed cull in jobs.
The Sigma engine that is built at Bridgend is exported to EU as well other global markets, including China and the United States, but is in the process of being phased out.
The new Dragon engine which will replace it will be made for Ford's European operations only, but demand is expected to be lower than for the Sigma.
Mr McCluskey said Ford had provided Unite with its plans for the next five years, embodied in which was a "significant drop in the number of jobs."
The 66-year-old said the plan was a sign of "worrying times" for the region, adding: "I'll be asking the Ford management when we meet them to explain this, to go into more detail."
He continued: "Our message is clear: Ford has got a duty to commit to the workforce in Bridgend and how important it is to the whole of South Wales."
He said Unite's message was getting through to Ford, but warned he would need to have some "hard words" with its management.
Mr McCluskey said he would be deeply involved in upcoming negotiations and would be fighting for a "continued future" for the region.
"It's [the message] certainly been getting through. We'll know whether they're listening or not when we continue with the discussions today," Mr McCluskey said.
He added: "The importance of Bridgend to the whole of the south Wales economy is well-known."
Mr McCluskey said: "There's no doubt that Brexit plays a role in this and that's why we're in discussions.
"The good news is, of course, this isn't happening next week. There's a three-year period in terms of the transition from the Sigma engine to the new Dragon engine."
He added: "Let's hope we can secure some kind of deal that gives a future to the Bridgend plant.
"We'll be fighting for their future and the future of this community."
Meanwhile, Theresa May has said the Government is continuing to talk to the car giant over the threat of job cuts.
Mrs May said she wants to help prolong the car-maker's success.