An army depot in Gloucestershire could close with 400 jobs relocating under plans by the Ministry of Defence.
The Defence Storage and Distribution Agency (DSDA) in Ashchurch maintains and repairs military vehicles.
The MoD said the depot's work could be transferred to the Defence Support Group (DSG) in Donnington, Shropshire.
The GMB union said it would be very difficult to replace the highly skilled workforce who deal with sophisticated and complex vehicles.
"This would also have a massive impact on the local community as DSDA is one of the major employers in the area," said GMB officer Wayne Powell.
The union said the DSDA had a "fantastic track record" in ensuring vehicles were highly maintained for troops in conflicts including the Falklands, Gulf, Afghanistan and Iraq and is urging the MoD to "rethink its position".
A MoD spokesman said: "Following a review by the MoD and the Treasury to look at improving efficiency across defence, we are looking to transfer responsibility for DSDA's vehicle maintenance and repair facilities at Ashchurch to the Defence Support Group.
"Trade unions are fully engaged in this process."