Hampshire and Essex-based electronics firm SELEX Galileo says measures have been ongoing for several months to try to cope with a decrease in orders.
But the company said to ensure it remained competitive, it had been forced to announce a planned reduction in the workforce of up to 25 per cent at the Basildon and Portsmouth sites.
The company has begun collective consultation with elected employee representatives.
The company will also hold individual consultations with employees affected. Employees currently on training programmes, such as technical and business apprentices, industrial placements and graduates are not affected by the announcement.
Electronics firm SELEX Galileo has announced it will cut up to 25 per cent of the workforce at its sites in Basildon and Portsmouth.
The company employs 676 people across both sites, with 35 of those based at Portsmouth. It said it had faced "major challenges" throughout 2012.