Investment giant Melrose is planning to cut 270 jobs as it continues its bid to buy engineering firm GKN.
Union officials said workers at the Brush turbogenerator business in Loughborough which Melrose bought in 2008 faced losing their jobs.
The firm blamed lower global demand for turbogenerators, which provide electricity for the oil and gas industry.
Unite national officer Linda McCulloch said: “Unite will be fighting to save jobs and protect our members’ interests.
“This is a highly skilled workforce who contribute massively to the local economy and wider UK engineering and manufacturing sector.
“Offshoring their jobs will deny the local community of decent, well-paid work and further hollow out the region’s engineering skill base.
“Unite is far from convinced of the case behind this move and will be challenging the rationale during the consultation.
“Over the coming days and weeks, Unite will be giving maximum support to our members through this distressing time as we fight to protect jobs and skills.”