New evidence suggests that insects are thriving on the (Bt) pesticide that is engineered into Genetically Modified crops.

Genetically modified crops specially engineered to kill pests in fact nourish them, startling new research has revealed.

The research - which has taken even the most ardent opponents of GM crops by surprise - radically undermines one of the key benefits claimed for them. And it suggests that they may be an even greater threat to organic farming than has been envisaged.

It strikes at the heart of one of the main lines of current genetic engineering in agriculture: breeding crops that come equipped with their own pesticide.

Biotech companies have added genes from a naturally occurring poison, Bacillus thuringiensis (Bt). The engineered crops have spread fast. The amount of land planted with them worldwide grew more than 25-fold - from four million acres in 1996 to well over 100 million acres (44.2m hectares) in 2000 - and the global market is expected to be worth $25bn (£16bn) by 2010.

Drawbacks have already emerged, with pests becoming resistant to the toxin. Environmentalists say that resistance develops all the faster because the insects are constantly exposed to it in the plants, rather than being subject to occasional spraying as is the method used by organic farmers.

But the new research - by scientists at Imperial College London and the Universidad Simon Rodrigues in Caracas, Venezuela - adds an alarming new twist, suggesting that pests can actually use the poison as a food and that the crops, rather than automatically controlling them, can actually help them to thrive.

They fed resistant larvae of the diamondback moth - an increasingly troublesome pest in the southern US and in the tropics - on normal cabbage leaves and ones that had been treated with a Bt toxin. The larvae eating the treated leaves grew much faster and bigger - with a 56 per cent higher growth rate.

They found that the larvae "are able to digest and utilise" the toxin and may be using it as a "supplementary food", adding that the presence of the poison "could have modified the nutritional balance in plants" for them.

And they conclude: "Bt transgenic crops could therefore have unanticipated nutritionally favourable effects, increasing the fitness of resistant populations."

Pete Riley, food campaigner for Friends of the Earth, said last night: "This is just another example of the unexpected harmful effects of GM crops.

"If Friends of the Earth had come up with the suggestion that crops engineered to kill pests could make them bigger and healthier instead, we would have been laughed out of court.

"It destroys the industry's entire case that insect-resistant GM crops can have anything to do with sustainable farming."

Patrick Holden, director of the Soil Association, said it showed that GM crops posed an even "worse threat to organic farming than had previously been imagined". Breed- ing resistance to the Bt insecticide sometimes used by organic farmers was bad enough, but problems would become even greater if pests treated it as "a high-protein diet".