The UK Government is sponsoring a review of GM science
Thousands of National Trust tenant farmers have been asked by the conservation organisation not to allow the growing of genetically modified (GM) crops on its land.
Dozens of controversial GM trials taking place across the UK are due to end this summer.
Public consultation will then follow on whether these plants should be grown commercially.
The government is due to launch a £500,000 "public debate" in the coming weeks.
The request by the National Trust to it 2,000 tenant farmers to boycott GM technology has been welcomed by campaigners Friends of the Earth.
Tony Burton, the National Trust's director of policy and strategy, said his organisation were adopting a "precautionary approach".
"We have not used our land for the purposes of those experiments and we are encouraging our members, and others, to contribute to the public debate."
He added: "We had the question raised and we have given our view and it's not gone any further that that."
But he indicated it was possible the trust's stance could change in the light of more scientific evidence.
Most farm tenants of the National Trust have a statutory right of "freedom of cropping" so long as they follow good agricultural practice. The trust cannot simply ban its tenants from planting novel crops.
The Prince of Wales, the trust's president, has previously been vocal in his opposition to GM crops.
Last year he said: "To me it's absolutely essential that we operate husbandry and stewardship of this piece of Earth."
GM crops can be designed to be naturally resistant to pests. But environmentalists fear the novel plants could spread their genes to "organic" crops and weeds.
Friends of the Earth director Tony Juniper said the government "must listen to this growing opposition to GM crops and refuse to allow the crops to be commercially grown".
The results of the GM crop trials will be published in the summer followed by a government-funded independently run public debate in June.
There will also be a scientific review of GM and a study by the prime minister's Strategy Unit into the overall costs and benefits of commercial GM cropping.
The Department for Environment, Food and Rural Affairs website says the government is "neither pro nor anti GM crops... it recognises that they have both potential risks and benefits".