ITV News has learned that Ford has drawn up plans to cut 1,160 jobs at its engine plant in Bridgend over the next four years.
It's understood that Ford is responding to a fall in global demand and problems with efficiency - including restrictive work practices - specific to Bridgend.
The company expects headcount to fall from its current level of 1,760 to around 600 by 2021, mostly through natural attrition.
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.
Demand for the Dragon engine is expected to be lower than for the Sigma.
Bridgend also makes engines for Jaguar Land Rover however that contract is set to expire at the end of 2020.
As it stands, Ford suggests that engine production at Bridgend will reduce from around 650,000 engines a year today to around 125,000 by 2021.
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 Sigma engines made in Bridgend are exported to Germany, Spain and other EU countries where they are installed in Focus, Mondeo and Fiesta models.
In January Ford warned that the slump in the pound since the Brexit was having a negative impact on the company and could cost it as much as $600m
The government has committed to leading Britain out of the single market and it's not clear that we will remain in the customs union - which raises the possibility of Ford having to pay tariffs on their components as they move across its European operations at some point in the figure.
Tonight neither the Unite or the GMB union was available for comment.
In a statement, Ford told ITV News: "We continue to work with our union partners on an ongoing basis. Anticipated production volume of engines from Bridgend remains healthy in the upcoming years, with associated labour requirements expected to be similar to today's level."
It added "Bridgend must fulfil its commitment in terms of delivery, quality and cost of the products it manufactures and - just as in the case for every Ford plant around the world - winning new product contracts depends on the plant's efficiency and global competitiveness."
A Welsh Government spokesman said: “We are seeking an urgent, high level meeting with senior Ford executives to explore what further the Welsh Government can do to secure the future for manufacturing and jobs at the Bridgend plant."