Any move to introduce commercial planting of genetically modified crops should be coupled with long term monitoring of the impact, top scientists have warned.

Publishing its submissions to the government's GM science review, the Royal Society said the potential environmental impact should be closely scrutinised.

Ministers were urged to set out their plans for monitoring GM plants and ensure that decisions are based "on the best scientific information".

"If the decision is taken to allow commercial planting of GM crops, it is essential that regulators in both the UK and EU monitor the environmental impact to pick up any potentially beneficial or harmful effects over a long period," said Professor Patrick Bateson, vice-president of the Royal Society.

"It will not be enough to make best estimates at the start and then assume that everything will turn out as expected."

The Society said that a review panel, which is due to report in June, should be allowed to fully consider the results of the current farm-scale trials which are expected to be published later this year.

"These trials are the largest scale field study of how particular types of GM crop affect wildlife, and the review panel should make recommendations to government in the light of this research," said Professor Bateson.

But environmental campaigners said the Royal Society should have gone further.

Pete Riley of Friends of the Earth said that if scientists were concerned about the potential environmental impacts of GM crops they should oppose their commercial development.

"Long-term monitoring will not prevent damage that has already been caused. Biotech companies must not be allowed to turn our countryside into one huge outdoor experiment," he said.