General Electric is to cut 369 jobs at its aircraft engine maintenance plant in south Wales.
GE has been consulting with 1,400 staff at its site in Nantgarw, near Caerphilly, as the firm suffers from the drop of numbers in air travel.
The company, which makes jet engines for Boeing and Airbus, blamed the "unprecedented impact of Covid-19".
It said it remained focused on "preserving our capability to respond as the industry recovers".
"Today's news from GE is further devastating news for Welsh workers, Welsh manufacturing and the Welsh economy," said union leader Peter Hughes, the regional secretary for Unite Wales.
"Our members at GE and their families will be extremely worried about their futures today.
"Unite will stand completely behind our members and will not move an inch from our position that compulsory redundancies must be ruled out."
The union called on GE to pause their plans and "work with us to seek an urgent UK government sector deal for aerospace that could save these jobs".
The Labour MP for Pontypridd Alex Davies-Jones and her Senedd counterpart Mick Antoniw said it was an "absolutely devastating" announcement.
They said it was "particularly distressing" that the majority of GE's UK job cuts would fall on the Nantgarw site.
"These are highly-skilled, highly-paid jobs and will be extremely difficult to replace," they said.
It follows job cuts at British Airways in south Wales and the announcement of 1,435 redundancies at Airbus in Broughton in Flintshire.
Ms Davies-Jones and Mr Antoniw added: "The UK aerospace industry is in crisis and it is not acceptable for the UK government to simply sit back and shrug its shoulders.
"We will continue to work with Unite the union and GE management locally to do everything we can to ensure that as many jobs as possible are retained for the future and to support those who have lost their jobs in any way we can."