A fire broke out at the Chemring Countermeasures facility after the blast on Friday shortly after 5pm.
The factory is just a few miles from the Ministry of Defence’s Porton Down laboratory which has been investigating the novichok poisoning attacks.
The blaze has since been extinguished and the incident has been contained with no risk to the public.
Police and health and safety officers are investigating.
A Wiltshire Police spokesman said: “The ambulance service contacted Wiltshire Police and Dorset and Wiltshire Fire and Rescue Service just after 5pm today following reports of an explosion at the Chemring Countermeasures plant on High Post Road, Netton.
“Six fire crews were in attendance. Two were from Salisbury and the rest from Andover, Fordingbridge, Pewsey and Cranbourne and were supported by the incident command vehicle from Devizes.
“An initial fire on site has been extinguished. We have two casualties that have been identified.
“One of them sadly died at the scene and the other has been taken to hospital in a critical condition.”
No further details of the casualties were released and police were yet to inform next of kin on Friday night.
The spokesman added: “The incident has been contained and there is no risk to people in the local area.
“Wiltshire Police and the Health and Safety Executive will now be leading the investigation into the cause of the incident.”
On its website, Chemring Countermeasures says it produces hardware, including flares, chaff and decoys, to help protect military ships, aircraft and land platforms from guided missile threats.
It has also designed countermeasure systems for the Eurofighter Typhoon fighter jet.(Sky)