The government decided last night to press ahead with 96 new trials of genetically modified crops this spring but unexpectedly refused to segregate them from conventional crops, leaving farmers and environmentalists outraged.

However, ministers accepted the trials will contaminate conventional crops nearby, leaving themselves open to damage claims from farmers whose crops may be unsaleable as a result.

Pleas by the organic and environmental lobby to leave 3km exclusion zones around GM crops in order to protect conventional varieties were rejected. The ministry of agriculture has increased the maximum margin from 50 to 100 metres. Agriculture minister Baroness Hayman accepted this would contaminate conventional crops but only up to 1%.

This could still render many farmers' crops unsaleable since all major British supermarkets now refuse to take GM contaminated food - or even produce from animals fed on GM crops.

Organic farmers have warned Michael Meacher, the environment minister, and Baroness Hayman that they could be put out of business unless the field margins were increased to 3km. They reject the ministry's claims that the contamination would be 1% or less - the Scottish Crop Research Institute, in Dundee, says it could easily reach 5%

The Soil Association, which polices organic farming, had asked for the exclusion zone to avoid members' crops being cross-pollinated by bees or

wind-blown pollen.

John Holden, head of the association, said: "Contamination is inevitable."

The govenment hopes that GM oil seed rape plantings will begin in mid-March, sugar beet from the end of March, and maize from April. The purpose of the research is to check on cross-contamination of crops and the effect of GM crops on weeds and wildlife, including bees.