It is time to stop the onward march of genetic engineering, says Jules Pretty

This week ministers once again considered whether to ban the growing of genetically modified crops in Britain for up to three years. Such a moratorium would give scientists a chance to assess the risks posed by this new technology, crop by crop.

In the event the government decided not yet to go ahead with a ban. I think it should.

Genetic engineering is the transfer of new genes to plants or animals - allowing them to do things not possible with their own set of genes. It sounds exciting and some of the possibilities offer immense promise.

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Fruit containing the genes of dangerous viruses could be produced, which will work like a vaccination every time we eat a banana. Animals low in fat will be developed; chickens with no feathers and sheep that shed their fleeces automatically. More alarmingly, cloned sheep could mean we will soon have cloned humans. People's genomes could be engineered to make them tolerant to pollution and able to work in poisonous environments.

But genetic engineering will also bring new problems. Some of these are fundamental ethical issues - should genes really be transferred across species that, in the natural world, do not breed? Should human embryos be used for laboratory experimentation? Will people end up being divided into different genetic classes?

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Then there are worries about what we are doing to nature. Farmers can already grow crops that have been engineered to make them immune to weed-killers. Fish genes have been put into crops so that they can withstand frost. We have plants with a built-in resistance to viruses and nematodes; blue cotton and fruits that do not rot. And it is all happening so quickly.

What happens if genetically modified crops cross-pollinate with weedy relatives, producing superweeds? Antibiotic markers used to identify modified cells in the lab could spread to people, making the antibiotics we use less effective.

The first signs of trouble appeared last month. Novartis is the company that developed maize with a gene that makes tissue poisonous to the corn-borer insect. This puts immense selection pressure on pests, and ecologists have warned that resistance to the toxic tissue could spread very quickly. In September the company asked farmers to keep 40 per cent of maize as traditional seeds, so that insects do not develop resistance so rapidly.

A group of the government's nature watchdogs has asked for a four-year moratorium on the use of these crops in Britain. Some European countries seem set to slam on the brakes: France has recommended a two-year moratorium on GM rape seed.

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It is time to put the lid, at least temporarily, on genetic engineering. We need time to assess the risks, to make sure that proper safety mechanisms are put in place and to make ethical judgements. This will all take several years. A moratorium would help us do that, calmly and sensibly.

Jules Pretty is director of the centre for environment and society at the University of Essex.

* Do you think genetically modified crops should be banned? Email us on soapbox@thes.co. uk