Maydown Precision Engineering (MPE) in Londonderry employs 133 staff and supplies parts to major aircraft firms.
It was bought last month by Waterford-based Schivo Group, which said up to 15 jobs were "under consideration".
Unite said its members received letters "warning about unspecified numbers of job losses at the company, which raises serious concerns for the future".
Regional officer Philip Oakes said: "We are very disappointed that the new owners of Maydown Precision Engineering have decided to exclude the union from their plans for redundancies in the workforce.
"It is unacceptable and a breach of our existing recognition agreement that we only found out about the planned job losses when members came forward with letters from management."
Unite had said there was concern for the future of 140 jobs, but a spokesman for Schivo NI, formerly known as MPE, accused the union of "inaccurate and irresponsible behaviour".
"Schivo NI has not breached any existing agreement with any trade union and 133 jobs at the facility in Derry are not at risk," he said.
"Schivo NI recently acquired the business and assets of Maydown Precision Engineering and in line with restructuring the business, a small number of staff in roles that are not key to the growth and development of the business have been informed that the company is considering making their positions redundant.
"Between 10 and 15 positions are under consideration, although at this stage nothing has been decided."
He said the company was "committed to the long-term growth and development of the business in Derry".
An aerospace engineering company has rejected claims by a union that a large number of jobs are at risk.