Military police are investigating claims that British soldiers may have been involved in heroin trafficking in Afghanistan.
Officials said they were aware of "unsubstantiated" claims that troops were buying the illegal drug from dealers and using military aircraft to ship it out the country.
An inquiry has been launched focusing on British and Canadian service personnel at airports in Camp Bastion and Kandahar.
Meanwhile security has been tightened, with additional sniffer dogs being used as part of the crackdown at the bases.
Afghanistan is the source of 90% of the world's opium.
Some drug bosses in the war-ravaged country have implicated soldiers in the trade.
Last year the Sunday Times spoke to one dealer who said members of the military were the second largest buyers of heroin after foreign drug lords.
The newspaper was told: "The soldiers whose term of duty is about to finish, they give an order to our boss."
The dealer, named only as Aziz, added: "They are carrying these drugs in the military airlines and they can't be reached because they are military. They can take it to the USA or England."
A team of detectives from the Ministry of Defence's special investigations branch is believed to be heading the investigation into the claims.
An MoD Spokeswoman said: "We are aware of these allegations. Although they are unsubstantiated, we take any such reports very seriously and we have already tightened our existing procedures both in Afghanistan and in the UK, including through increasing the use of trained sniffer dogs.
"We regret any inconvenience this causes to our service personnel. Any of our people found to be engaged in trafficking of illegal narcotics will feel the full weight of the law."