Claims that British soldiers smuggled heroin out of Afghanistan were being investigated by military police last night.
Troops are said to have used Army planes to sneak shipments out of the country after buying from dealers.
Officials said they were aware of "unsubstantiated" allegations and an inquiry was focusing on British and Canadian personnel at Camp Bastion and Kandahar airports.
Security has been tightened, with more sniffer dogs after a dealer claimed military personnel were the second largest buyers after foreign drug lords.
Named only as Aziz, the dealer said: "The soldiers whose term of duty is about to finish, they give an order. They can take it to the USA or England."
The MoD said: "People engaged in trafficking will feel the full weight of law."