Take The Flour Back’s website bills the demonstration as a “nice day out in the country” to “share picnics, fun, and flour frolics”.
However, scientists fear an attempt will be made to wreck the experiment.
‘We cannot stop you’
In their letter, which is also being sent to other members of the scientific community and politicians, they wrote: “Dear Take The Flour Back, we have learned that you are planning to attack our research test site on 27 May.
“We know we cannot stop you from taking the action you plan, nor would we wish to see force used against you.
Therefore we can only appeal to your consciences, and ask you to reconsider before it is too late, and before years of work to which we have devoted our lives are destroyed forever.”
The scientists argue the moral case for their research, stressing that they agree agriculture should seek to work “with nature rather than against it”.
In a plea for common sense, they say: “You have described genetically modified crops as ‘not properly tested’. Yet when test are carried out you are planning to destroy them before any useful information can be obtained ..What you are planning to do is reminiscent of clearing books from a library because you wish to stop other people finding out what they contain.”
The letter is signed by seven scientists, including the trial leader Professor John Pickett.
‘Do the responsible thing’
At a recent press conference in London, the researchers told how they had inserted a peppermint gene into wheat so it released an aphid-deterring pheromone.
The smell is recognised by the aphids as an “air raid warning” of an attack by a predatory parasitic wasp.
Scientists hope it will keep aphids away and attract wasps to further control the pests.
Eight plots of land, each six metres square, have been planted with the GM wheat.
A spokeswoman for Take The Flour Back, who called herself Helen, said members may take action to remove the crop if the experiment is not halted.
“We’re asking the scientists to do the responsible thing and stop this trial,” she said. “If they don’t, we’re going to have to do the responsible thing ourselves.”