The aerospace firm, Bombardier, is putting its Northern Ireland operation up for sale as part of a reorganisation of the business.
The Canadian aircraft manufacturer employs about 3,600 people across several locations in Northern Ireland.
The company said it would be working closely with employees and unions, through any future transition period.
Unions said it caused “uncertainty” for workers at Northern Ireland’s biggest manufacturing employer.
It is selling off its aerospace operations in Belfast, Newtownabbey, Newtownards and Dunmurry. The company’s Moroccan operation is also being sold off.
In a statement, Bombardier said it was consolidating all aerospace assets into a “single, streamlined and fully integrated business”.
The statement added: “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.”
It said Bombardier was committed to finding the right buyer.
It added: “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.”
‘Waiting for an announcement’
Business Secretary Greg Clark spoke to representatives of the company before the announcement was made.
“The Belfast plant is one of our most important aerospace facilities in the country and a vital asset in the UK’s leading aerospace sector,” he said in a statement.
“The government will work with potential buyers to take this successful and ambitious business forward.”
Speaking on BBC News NI’s Talkback programme, Gavin Robinson, MP for East Belfast, said that, in recent years, Bombardier workers have not been able to “get a break”.
He said that “government has a role to ensure that all avenues are explored” in relation to the sale.
Analysis: John Campbell, BBC News NI economics and business editor
It is not yet clear who could buy the Belfast operation but it may be attractive to global engineering firms who are ‘Tier 1’ aerospace suppliers.
Industry watchers point to firms like Spirit Aerosystems or GKN.
The Belfast plants don’t just make parts for Bombardier, they also supply external customers such as Airbus.
Bombardier Belfast director, Michael Ryan, previously said the Belfast factory would be capable of functioning as an outside supplier to Bombardier’s business-jets division.
Susan Fitzgerald, the regional co-ordinating officer with Unite trade union, said that the Bombardier workforce have been “bracing for a shock announcement every morning”.
“The sale causes significant uncertainty for workers and members,” she said.
Stephen Kelly of Manufacturing NI warned that, between workers and Bombardier suppliers, the sale will have a direct impact on 12,000 jobs in the Northern Ireland economy.
“It’s deeply worrying for the suppliers… and it is deeply worrying for the workers,” said Mr Kelly.
Bombardier, which is based in Montreal, has more than 68,000 employees in 28 countries.
Last month, it slashed its full-year revenue forecast from $18bn (£13.7bn) to $17bn (£13bn) due to timing of aircraft deliveries, production challenges in its train-making division and unfavourable currency conversions.
The rail unit is meant to generate $10bn (£7.6bn) but Bombardier has cut its full-year revenue forecast for the division by almost 8% to $8.75bn (£6.7bn).
Bombardier’s president and chief executive Alain Bellemare said that the company expected to meet its aircraft delivery and financial performance targets for the year in its aerospace businesses.