The union aims to ensure redundancies are voluntary
Union officials fear the possibility of up to 800 job losses at the Forensic Science Service, as the government consults on transforming the body.
The Prospect union said managers, scientists and computer staff could lose their jobs, and sites could close.
It said it would scrutinise cuts to ensure FSS did not lose needed skills.
The Home Office said "wide-ranging transformation" was vital to the survival of the service, which analyses crime evidence in England and Wales.
A department statement has said the FSS intends to "move to a new business model, delivering the same integrated service, more quickly and efficiently, with a reduced but more targeted workforce, and potentially working from a reduced estate".
Prospect said it was shocked at the scale of cuts, saying up to 40% of the workforce could go.
General secretary Paul Noon said: "We will be scrutinising where these job losses fall to ensure this is not just an exercise to reduce the headcount, leaving FSS denuded of the skills it needs."
The union also said it would resist any attempt to impose "significantly worse" terms and conditions on remaining staff.
A Home Office spokesperson said: "The forensics market is changing with increased competition, reduced business volumes and a higher cost base.
"It is right that the FSS should put itself in a sustainable position for the future."
The statement added: "As part of this, the FSS has today commenced a consultation with its staff and unions on the implementation of a wide-ranging transformation plan which is vital to the long term survival of the company."
The service analyses crime scene evidence for England and Wales police forces, dealing with more than 120,000 cases a year.