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MUCH of the British countryside may be off-limits to genetically modified crops before the government even decides if they can be grown commercially.

The reason is organic farming, which is becoming ever more popular in Britain. If it continues to boom, many potential GM crop sites will disappear, a computer model suggests. But rather than one type of crop replacing the other, the land will instead be given over to the buffer zones designed to keep organic and GM crops apart.

For instance, GM maize cannot be planted within 200 metres of organic crops. This buffer zone is designed to prevent the GM crops cross-fertilising with their neighbours, 鈥渃ontaminating鈥� produce. At the moment, only 3 per cent of agricultural land is organic, but this is growing fast. In 2000, it jumped by a third and consumer demand for organic produce is also rising.

So Joe Perry, an ecologist at the Institute of Arable Crops Research at Rothamsted in Hertfordshire, investigated what effect this growing demand would have on land use. His model maps an agricultural region with organic farms scattered among conventional agriculture. Around each organic farm is a no-go area for GM maize. The model can be adjusted to investigate the effect of changing the size of the buffer zone or increasing the area covered by organic farms.

Perry wondered what would happen if a fifth of the agricultural land was given over to organic crops. The present separation of 200 metres would exclude GM maize from about 30 per cent of the remaining area. But the model showed that if the buffer zones were extended to 600 metres then three-quarters of sites would be off-limits.

That looks increasingly likely. With more arable land now under organic cultivation, the government is considering enlarging the size of the buffer zones. At present, produce is deemed to be organic if GM crops contaminate less than 1 per cent of it. But last year, the environment minister Michael Meacher hinted that this might be cut to only 0.1 per cent.

Perry says it is unclear how the government arrived at this new figure. But the buffer zones would have to be hugely increased to meet this target. 鈥淚f organic production expands significantly, it is difficult if not impossible to see how GM crops can coexist with their organic counterparts,鈥� says Patrick Holden, director of the Soil Association, based in Bristol.

But Chris Leaver, a GM expert from Oxford University, believes that organic farms can never take up that much of the countryside, as they benefit from the pesticides sprayed onto neighbouring fields. 鈥淥rganic agriculture thrives because it has a cordon sanitaire of conventional crops around it,鈥� he says. If conventional crops fall in number, the yields of organic crops will drop, making them less economic.