Bombardier is to sell its aerostructures business in Belfast - and GMB Union is demanding assurances over jobs.
A statement by the Canadian manufacturing giant confirmed rumours which had been circulating at the plant in Northern Ireland.
The company admits the announcement may "cause concern" for employees.
A statement issued in Montreal said: "As the company moves to optimise its global manufacturing footprint, Bombardier will pursue the divestiture of the Belfast and Morocco aerostructures businesses. These are great businesses with tremendous capabilities."
Meetings were being held at the Belfast site on Thursday when more details were expected to be given.
A statement by Bombardier in Belfast said: "Our sites in Belfast and Morocco have seen a significant increase in work from other global customers in recent years.
"We are recognised as a global leader in aerostructures, with unique end-to-end capabilities - through design and development, testing and manufacture, to after-market support. Bombardier is committed to finding the right buyer - one that will operate responsibly and help us achieve our full growth potential.
"We understand that this announcement may cause concern among our employees, but we will be working closely with them and our unions as matters progress, and through any future transition period to a new owner.
"There are no new workforce announcements as a result of this decision, but our management team will continue to drive ongoing transformation initiatives to improve productivity and increase our competitiveness, to give more weight to our unique value proposition to potential buyers."
GMB union organiser Michael Mulholland said: "GMB demands reassurances over the future of hard-hit Bombardier workers in Northern Ireland.
"Our members - and their families - have already suffered a terrible year. "After months of uncertainty following Trump's tariffs, they were then hit with news of 490 job losses, and now this.
"Bombardier jobs are absolutely vital to Northern Ireland's economy and it's time workers were treated with the respect they deserve.
"GMB will be meeting reps today and over the next few days to discuss next steps."