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The storage of 28kg (61lb) of Semtex at the site near Lyon was “not usual and certainly not authorised”, Xavier de Fuerst also told the AFP news agency.

A search by anti-terrorist officers is under way, and the manager of the site has been suspended pending an inquiry.

Semtex has been favoured by terrorists for being very difficult to detect.

The depot, in a disused 19th Century fort at Corbas, is used by a civil defence unit charged with the job of blowing up bombs and ammunition left over from the two world wars.

Mr de Fuerst said the site had been bought by the interior ministry in 2005, and that existing security measures had fallen into disrepair.

He said that the storage of the explosives had not been authorised until work to replace security cameras and reinforce the entrances – scheduled for 2009 – was complete.

He said that neither the police nor local authorities were aware that the Semtex was being stored there.

The mayor of Corbas, Thierry Butin, told local media: “I am extremely surprised and angry that from one day to the next you can store products as dangerous as this, without any protection.”

‘Security failings’

Police said detonators were also missing and that they were treating the theft “very seriously”.

Police said the discovery that the Semtex was missing was made on Friday but admitted the explosives could have been taken up to a week ago.

In a statement released late on Friday the interior ministry said “security failings” had made the theft possible.

Semtex, which was first made in the Czech Republic, is used in mining and demolition work.