The explosion at the Chemring Countermeasures factory of the British contractor also injured another person who was taken to a hospital, the company said in a tweet.
The site was evacuated and the incident was brought under control, the company said, adding that the defence equipment maker has launched an investigation into the cause of the incident which occurred on Friday evening.
British media had earlier reported on the explosion citing the Wiltshire Police.
"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," a representative of the Wiltshire Police was quoted as saying by the BBC.
Reuters could not immediately reach the police force for comment.
Chemring Countermeasures makes products to safeguard military ships and aircraft from attack.