Local police stations in Lincolnshire, Nottinghamshire and Hertfordshire were issued with warnings as the locations of the four farm-scale sites were posted on the Internet.
In what Downing Street said was proof of its commitment to openness on the controversial technology, the precise grid references of the farms were listed on the Department of the Environment, Transport and the Regions' (DETR) website.
To the alarm of green campaigners, the DETR also announced a big expansion of the experimental tests, revealing it would announce a total of 75 new test sites next year.
At present, there are just six farm-scale trials in operation to test the impact on the countryside of GM oilseed rape and maize developed by biotech giants Monsanto and AgrEvo.
But from next spring, there will be another 75 farms involved in the trials, each with permission to plant up to 25 acres of the crops - much bigger areas than presently used.
Greenpeace, which destroyed a field of modified maize in Lyng, Norfolk, last month, refused to rule out similar "direct action" against the new sites. Its executive director, Lord Melchett, arrested after the incident, said he would give no assurance that the group would leave them alone. However, he said his own bail conditions prevented him personally from going near any GM site until his trial.
Genetix Snowball, a smaller, more radical group whose members have been arrested for uprooting GM crops, also warned it may be involved in similar demonstrations. "Our campaign of non-violent, civil responsibility will continue and that includes removing GM crops from the environment," a spokesman said.
Scores of underground green groups are understood to be preparing to send teams to sabotage the trial sites.
The four farm-scale trials of GM oilseed rape will begin this autumn in Bingham, Nottinghamshire, Hemel Hempstead, Hertfordshire, and in Lincolnshire at Market Rasen and Glentham.
A Downing Street spokes-man said the trials were being pursued in greater numbers to ensure that the GM debate had access to the full scientific facts. "Jack Cunningham has said we would have to monitor the situation but the view is that people are entitled to know where these sites are," he said.
The spokesman stressed that reports of special GM police were "wide of the mark". "Any public order situations are to be dealt with by the local constabularies," he said. "These farmers can be reassured that the police will be well aware of where these sites are and will be on hand to deal with any difficulty."
But Roger Turner, head of the British Plant Breeders' Association, warned the Government to rethink its policy of openness in the face of the environmentalists' threats.
"I'm all in favour of openness and transparency but obviously there are going to be limits if people abuse that position," he told BBC Radio 4's Today programme.
"We've said to Government that they should reconsider the actual specific details and think of other ways to maintain the openness."
Bob Fiddamen, one of the farmers who announced yesterday that he would grow GM herbicide-tolerant oilseed rape on his land in Hemel Hempstead, said it would be "very arrogant" of protesters to destroy his crops.
"The irony is, this herbicide is less damaging to the environment than the ones we use now. This test could have many environmental benefits."
Both Friends of the Earth and Greenpeace called the trials "a farce" that ran counter to public opinion and attacked the Government for failing to consult local people.
Liana Stupples, campaigns director for Friends of the Earth, said there was a risk of cross-contamination of nearby organic crops or wild plants. "The Government's policy is to plant field after field with GM crops and see if anything goes wrong. This isn't science - this is creeping commercialisation."
Lincolnshire police said they would not act as private security for any of the trials, but would "not hesitate" to intervene if there was a threatened breach of the peace.
Leading article, Review, page 3