About 1,200 Wrightbus workers have been made redundant after the company entered administration.
Just 50 jobs will be retained at the firm - the last UK-owned bus manufacturer - administrators said.
The company had suffered cash flow problems and had sought investment or a new owner.
Talks with two potential buyers of the firm, best known for building the New Routemaster, known as the "Boris Bus", failed to reach a conclusion last week.
Deloitte, the firm's administrator, said the lack of a buyer for Wrightbus had caused the redundancies.
The Wright family, who founded Wrightbus in 1946, said "global changes from diesel to electric in bus technology have caused a sharp decline in demand for buses in the UK".
In a statement, the family said that it moved work from its facility in Malaysia to Ballymena in a bid to secure local jobs.
"These factors have resulted in significant losses at Wrightbus which our family have been covering for a over a year."
But a statement added: "It simply became impossible to sustain that level of support."
At a press conference on Wednesday afternoon, union officials called for the immediate intervention of the British government.
They said they sought meetings with Prime Minister Boris Johnson and NI Secretary Julian Smith, who they said they must "stand up for British industry".
Jackie Pollock, Unite's regional secretary, added "the government must intervene to save jobs and skills".
"The administrator will need to find someone of calibre and of real standing who has the wherewithal and the skill to take this company on," he said.
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