More than 200 aviation workers facing redundancy when work is transferred from RAF St Athan in the Vale of Glamorgan have held a march in Cardiff.
The Amicus union want a change of heart by the UK government
Hundreds at the Defence Aviation Repair Agency (Dara) have already received "jobs at risk" notices.
Tuesday's march to the Welsh assembly building came as their union, Amicus meets First Minister Rhodri Morgan.
Workers want the UK government's decision to move aircraft repair work to be shelved.
More than 500 jobs have already gone at St Athan, and Amicus said it expected redundancy notices to be issued at St Athan this month.
It follows the decision to switch work to another RAF base in Norfolk.
The union fears the plant will close if plans to transfer work back to the MoD go ahead.
The £80m facility, which was officially opened in April and employs about 1,300 people, has already lost 500 posts after losing a number of repair contracts.
Amicus officials met Mr Morgan and the Economic Development Minister Andrew Davies, at 1230 BST on Tuesday to talk about options available.
The union wants to lobby senior UK government ministers to change their decision to move the work back in-house.
Union delegation leader Cath Speight, the Amicus regional secretary for Wales, ahead of the meeting, said: "We and Welsh ministers cannot allow this closure to happen but time is running out. Wales desperately needs to retain these skilled jobs".
Dara at RAF St Athan had contracts to maintain Tornados and Harriers
Referring to the multi-million pound "superhanger", Ms Speight said: "This decision will be disastrous for the MoD and taxpayers who have already invested £80m in what will effectively be an enormous white elephant if this goes ahead."
The rally at the assembly building in Cardiff Bay was addressed by Ms Speight, as well as John Smith and Jane Hutt, MP and AM respectively for the Vale of Glamorgan
Dara workers and union offcicials also tore up a giant cheque for £80m.
Hundreds of jobs have already been lost since Harrier and Tornado aircraft maintenance work was moved away from St Athan last year and Amicus said the cuts will close the plant by 2007.
But the UK government has said switching maintenance work away from Dara - a civilian arm of the MoD - to the RAF would mean better value for money.