The move and the withdrawal of Jaguar aircraft from service will result in 500 jobs losses at St Athan and places a question mark over the future of the site, despite investment by the Welsh Development Agency in Project Red Dragon, intended to make St Athan a world-class aviation centre.

Prospect says the decision is bitterly disappointing for staff because they have risen to every challenge put before them in recent years to provide a public sector benchmark for defence aviation.

DARA has four sites and employs 3,800 staff – all highly skilled aviation engineers – and was set up to cut the cost of maintaining and repairing the UK’s military aircraft. St Athan is the main base and currently employs around 2,000 staff. In March 550 jobs were axed when it lost a contract to upgrade Harrier jets to the RAF.

Prospect national secretary Mike Clancy said: "Having invested time and money in developing the facilities at St Athan and across DARA, MOD now seems intent on dismantling it all. It is a big mistake.

"The union has made it clear to MOD that if it persists with this bizarre approach it will lose a public sector capability that will leave it vulnerable to private sector contractors and destroy a crucial skills base that cannot be replaced. The department must act now to give DARA a strategic direction for the future. If not St Athan faces closure. The local community will be devastated."

Prospect has challenged the assumptions in the investment appraisal that supported the proposal to transfer the Tornado work to RAF Marham. The union says it has deep concerns over the ability of RAF personnel to undertake the work and has called on the department to explain how the operational requirements of the Tornado platform, and the savings claimed, will be delivered.