Nearly 400 jobs will be lost at Rolls-Royce's plant in Ansty, near Coventry, as the aerospace giant seeks to reduce costs in response to government spending cuts.
The plant employs 800 to repair and refit engines for planes and boats, of which 378 are to be made redundant.
A Rolls-Royce spokesman said he hoped the company can avoid compulsory redundancies and is in consultation with trade unions.
Ian Waddell, an officer at the Unite union, said: "The underlying reason for the job losses is the Government's defence spending cuts."
The site is also a base for civil aerospace business which is not expected to be affected.