Military police are investigating claims that British soldiers may have trafficked heroin from Afghanistan.
The Ministry of Defence said they were aware of "unsubstantiated" claims that troops were using military aircraft to ship the drug out of the country.
The inquiry is focusing on service personnel at airports in Camp Bastion and Kandahar.
Security has been tightened, with additional sniffer dogs being used as part of the crackdown at the bases.
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."
Drugs and weapons
BBC Defence Correspondent Jonathan Beale said the Royal Military Police's Special Investigation Branch launched their investigation before the allegations were reported in the press.
Our correspondent added the increased security measures - such as the use of sniffer dogs at Afghan and British airbases - were introduced last month.
Afghanistan is the source of 90% of the world's opium.
The multimillion dollar trade in poppy production is used to fuel the insurgency.
It allow militants to purchase weapons with which they then attack the Afghan government and international forces, destabilising the region.
According to a 2008 UN report, 98% of the country's opium is grown in just seven provinces where there are permanent Taleban settlements and where organised crime profits from the instability.
