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The decrease is a sign of farmers' unwillingness to deal with the adverse publicity GM crops attract and shows that their development is being hampered by public opinion.

But Dr Penny Maplestone, of Scimac, a grouping of plant breeders, seed companies, farmers and agro-chemical companies told BBC News Online: "The number has decreased, but the area planted has not and may have increased slightly. The crops are now being grown on a larger scale."

She admits that farmers find it hard to take part in trials because of the pressures put on them by local attitudes and, in some cases, direct action where their crops are destroyed: "Lots of farmers do not want that stress and hassle."

"But this won't affect the development of GM crops in the UK, though it will make it more difficult," she adds.

Frightened farmers

Norfolk has seen the largest drop in GM trials, with the number of participating farms falling from 13 to seven. A green pressure group, the Norfolk Genetic Information Network, sent a questionnaire to farmers and said that many were concerned about local reaction to GM trials.

BBC reporter Tom Fielden: "So much public concern"

Tom Fielden, a reporter for BBC Radio's Today programme, said: "It looks as if many farmers are beginning to say to themselves, 'Do I really want to be the one farmer locally going ahead with the new technology when there is so much public concern about the introduction of these crops into the countryside?'"

The Co-operative Wholesale Society farms the largest acreage in the UK, and was the first to pull out of GM trials on their land. Their Head of Corporate affairs, Bill Shannon, says: "Clearly at the moment there is a high degree of concern about public opinion over these trials."

Mr Shannon says that, ironically, trials are the only way to overcome these concerns and that all parties should hammer out an agreement about how they should be conducted.

Trial and error

It is generally accepted by the opponents of GM crops that trials are necessary and there is considerable agreement about how trials should be changed to be acceptable to them.

The main concerns are that the trials:

• Are not adequately isolated from the countryside around
• Serve the interests of the companies developing the crops not the consumer
• Will not produce useful scientific results

Norman Baker:" It is not enough for the Prime Minister to smile and tell the public everything is all right."

Norman Baker, the Liberal Democrat spokesman on GM crops, says there is a chronic lack of public confidence: "The trials are seen to serve the interests of a small number of multinational companies and not the interests of the consumer, farmer or environment."

And Dr Arpad Puzstai, the scientist whose research sparked the current GM debate, says: "The main problem with the [trials] is that we don't know how these experiments are set up. We thought our experiments were properly set up, but at the end they were still rubbished."

But Dr Maplestone says: "It is not a fair comment to say the trials are not independent.

"The companies have to pay an entry fee to put their plants into the trials but they are not then involved to any extent and cannot influence the outcome. In the farm-scale trials, the ecological monitoring is paid for by the government," she says.

"Jury still out"

These concerns, and in particular the possibility of a GM trial compromising the organic certification of crops, led the trustees of a partly organic farm to end a GM rapeseed trial at the weekend.

The government has said that the "jury is still out" on GM crops but has insisted that the procedures in place adequately safeguard the environment and consumers.