Interior Minister Ivan Demerdzhiev said the authorities “are not ruling out outside interference” in the incident in central Bulgaria. No-one was injured in the blaze that triggered a series of explosions.

EMCO company, whose weapons are used by Ukraine against invading Russian forces, said the fire at its site was sabotage and that human error was “completely ruled out”. It did not say who might be responsible.

In a statement, EMCO said the storage facility that was burned this time - near the central village of Gorni Varpishta, some 170km (106 miles) east of the capital Sofia - had not been used for months and was inspected just 20 days ago, when full compliance with all regulations was confirmed.

It cited a number of similar incidents at Bulgarian defence firms since 2011, saying that the latest was “no exception to the series of acts of sabotage against the Bulgarian defence industry”.