"It's not trivial," says Christopher Leaver, a professor of plant science at the University of Oxford.

You need labs, an understanding of genetic technology and lots of hi-tech equipment. And it takes a long time. Which makes reports from Colombian police this week that they have found GM coca plants in northern Colombia which produce more cocaine seem suspect.

In theory, if you understood the way in which a certain gene makes the coca plant produce the cocaine alkaloid, you could modify it to be more productive. But not all plant species can easily be modified. "I'm not sure if the coca plant is transformable," says Peter Meyer, a plant geneticist at the University of Leeds.

"It is more likely to be selective breeding," says Leaver. "I would be extremely doubtful that it had been produced by genetic modification."

In any case, Meyer says that if GM technology was being used, there would be easier ways to produce more cocaine alkaloid. "If somebody really wanted to do this, there is no need to do it in the cocoa bush," he says. "You could do it in another plant which is more convenient to grow." Local toxicologists agree: they blame extra fertiliser.