Thirty sites around Britain will be sown with genetically modified oilseed rape next month in the latest stage of the Government's three-year experiment to measure the effect of GM crops on the environment.

Thirty sites around Britain will be sown with genetically modified oilseed rape next month in the latest stage of the Government's three-year experiment to measure the effect of GM crops on the environment.

Planting at the sites – 26 in England and Wales, and four in Scotland – will begin next month. Protesters are likely to try to destroy some of the crops. Their locations were revealed in a government notice yesterday and they will also be advertised in local newspapers, as dictated by European law.

Refusing to bow to pressure from environmental groups, the Government said it would not widen the gap between trial fields and other crops. The required separation from GM varieties will remain at up to 100 metres for standard crops, and 200 metres for similar organic crops.

The environmental pressure group Friends of the Earth complained that the separation was insufficient to prevent pollen from the GM crops reaching conventional crops, and possibly creating hybrid plants.

In the past year there have been 13 incidents of GM protesters invading fields and tearing up crops. But the Government and the crop industry decided that public trust would be eroded if the locations were kept secret.

"The companies involved have not asked for the locations to be kept secret," said a spokesman for the Department for the Environment, Food and Rural Affairs (Defra), which is now in charge of the process.

Scientists are monitoring the trials to see whether growing GM crops affects the normal balance of animal and plant life in the surrounding area. In 2003 they will consolidate and publish the research from three different sorts of crops – GM oilseed rape, GM sugar beet and GM maize – to consider whether commercial planting of GM crops should be allowed in Britain.

Pressure groups said the trials already had the potential to have a widespread impact. Peter Riley, of Friends of the Earth, said: "Oilseed rape pollen travels miles and neighbouring crops could be cross-pollinated, leaving farmers the problem of selling a contaminated crop."

Defra admitted that the separation of 200 metres did not completely rule out the possibility that GM pollen would reach plants on organic fields. That could lead to organic farmers potentially selling food contaminated with GM elements.

Mr Riley also criticised the Government for lack of proper consultation over this latest round of trials. "Government ministers have been paying lip service to consultation but when it comes to the crunch, it simply does not happen," he said. "The farm-scale trials should not proceed until the whole community has agreed how or if genetic pollution can be controlled and what is or isn't acceptable."

Mr Riley suggested that the new round of trials taking place in locations used previously showed fewer farmers wanted to be part of these "unwelcome and unpopular" GM trials. But Daniel Pearsall, of the seed planters' association, Scimac, denied the claim.