Massive stockpiles of raw opium grown in Afghanistan are being moved out of the region in anticipation of military strikes, western sources say.
There is thought to be an estimated 3,000 tonnes of the drug - enough to produce 300 tonnes of pure heroin with a street value of Stg£20bn - in the country.
A Downing Street spokesman says there is evidence of a sudden movement of opium out of neighbouring Pakistan where it was being stockpiled.
The disclosure will prompt fears that the West is about to be flooded with a glut of cheap heroin by Afghanistan's Taliban rulers.
The Taliban are known to have used the profits of the trade in drugs to fund their military activities.
Osama bin Laden - named by Britain and America as the prime suspect behind the September 11 atrocities - is also said to be closely involved in the Afghan drugs trade and has his own substantial stockpile of opium.
Opium grown in Afghanistan is thought to account for 95% of the heroin reaching Britain and around 75% of the total supply worldwide.
Although the Taliban last year finally banned the growing of opium poppies, there have been unconfirmed reports that they have threatened to lift the ban if America and its allies go ahead with military strikes.