Some Colombian drug growers are using genetically modified coca "trees" to boost cocaine production dramatically, government officials say.
Regular coca leaves only grow to around waist height
Anti-drug operatives say they found new strains with yields eight times higher than normal coca plants.
Higher yields could help explain why cocaine prices have stayed low despite US and Colombian air attacks on farms.
Colombian scientists and US officials expressed doubts, claiming extra growth could be achieved using fertiliser.
The coca "trees" can stand over 2m tall (6ft 6in) and produce four times as much of the alkaloid active in cocaine, according to a dossier seen by Britain's Financial Times newspaper.
Although official Colombian figures claim that the area under coca cultivation has halved since 2000, evidence suggests that coca planters have managed to maintain a net level of cultivation.
German Manga, an assistant to the Colombian vice-president, told the BBC that planters were using new and sophisticated technology to maintain their levels of production.
The leaked dossier said a new variety of coca plant had been discovered by anti-narcotics officers in the remote Sierra Nevada region of northern Colombia.
"In their search for greater profits, drug traffickers appear to have entered the world of genetically modified crops," the dossier said.
President Uribe has pledged to defeat "narco-traffickers"
Among the coca plants judged to have been genetically enhanced is one variety which grows up to 2.7m (9ft) tall - double the usual size.
Foreign agronomists have helped the coca growers to develop the new strain of plant, which is resistant to many commonly used herbicides and can yield as much as four times the regular concentration of cocaine, the Financial Times said.
But a Colombian toxicologist, Camilo Uribe, told Reuters news agency there was no evidence that the plants had been genetically modified.
The coca plants' excessive size could be because of "an excess of fertiliser", Mr Uribe said.
A spokesman for the US embassy in Bogota said there was "no scientific proof" that transgenic coca had been developed, although rumours of its existence were rife.
The BBC's Jeremy McDermott in Bogota says the new discoveries threaten to undermine the gains made on the war on drugs in Colombia.
If the drugs trade cannot be destroyed, then the warring factions that earn up to $1bn a year from narcotics will be a lot harder to defeat, our correspondent adds.
Under the government of President Alvaro Uribe, elected in 2002, Colombia has attempted to crack down on cocaine production across the country.
He is an enthusiastic supporter of Plan Colombia, a US initiative to train Colombia's security forces and provide them with equipment and intelligence to tackle drug traffickers and destroy coca crops before they can be processed into cocaine.
Under Plan Colombia the country has become the world's third largest recipient of US military aid.