An investigation into the number of GM trial sites registered with the Department of the Environment this year to grow GM crops - such as herbicide-resistant varieties of oilseed rape, sugarbeet and winter maize - has found only 148, compared with 309 in 1998.
Jonathan Matthews, of the Norfolk Genetic Information Network, said yesterday that many of the sites taken out of use in the past year were in his county.
He said: "When farmers sent replies to us we were very surprised at the range of concerns they expressed."
He thought that the farmers had been disappointed by yields of the modified crops and their low marketability.
However, two big biotechnology companies, Monsanto and AgrEvo, who sponsored half of the registered trial sites last year, disputed the claim yesterday. They said the reason the number had fallen was that their programme of testing was almost complete.
"The reason that the number of trials we are doing has fallen isn't some inability on our part to get farmers to co-operate," said Clive Rainbird, a spokesman for AgrEvo. "We have been doing small-scale trials on these crops since 1993, and those have provided all the necessary data for the government departments involved. The amount of data required is dropping off - so the number of trials we are doing has fallen from 64 last year to 45 this year."
Marketability was not an issue, he said, because none of the crops was being grown commercially. Farm trials had to be completed before the Government would license the crops for commercial growing.
Monsanto offered the same reason for the number of its trial sites falling from 100 last year to 38 this year.
"Many of those sites were demonstration sites, to show partners in industry what the crops actually looked like," said Tony Combes, a spokesman. "Having seen them one year, they don't need to see them again."
However, the figures will be used in the bitter debate over GM crops, which has already been heightened this week by the news that a farmer in Wiltshire had to destroy a 25-acre trial of herbicide-resistant oilseed rape on the orders of his farm's trustees. They had faced a threat of losing the income from their adjacent organic farming fields amid fears of GM contamination.
The farmer, Fred Barker, was strongly in favour of both the trial and GM crops, but was obliged to spray the field with paraquat weedkiller to destroy them. The Soil Association said the chemical had been used in a way that did not affect the farm's organic crops.
Biotechnology companies are now seeking farmers who will conduct trials of winter-sown oilseed rape, but landowners appear to be increasingly reluctant to allow tenant farmers to take part, fearful of trespass by protesters against GM. It emerged yesterday that Robert Kenyon, who owns a farm near Arborfield, Berkshire, had overruled his tenant farmer and withdrawn permission last week for the planting of a trial site of three acres of GM forage maize developed by Monsanto. The company said the trial had merely been "postponed".
Such disputes are likely to become more common in the months ahead.