The group, whose patron is Prince Charles, said they would blacklist organic produce from Lushill Farm at Hannington, Wiltshire for five years unless Capt Barker destroyed the official Government trial.
The ban would have cost Capt Barker at least pounds 50,000 in lost sales, so the weedkiller Paraquat was sprayed on the trial site, one of seven in Britain intended to determine whether GM crops cause environmental damage when grown on a commercial scale.
Yesterday the Government and the biotechnology industry insisted the other six farmers in the trial are determined to carry through with their GM trials, and more GM crops are planned for winter planting.Those against GM crops are pushing for a five-year moratorium on planting, and the biotech industry is relentlessly following the Government's rules hoping to win approval for commercial-scale growing.
The ultimatum that led to the GM trial's destruction was delivered on 21 April. The Association director Patrick Holden told Capt Barker and John Messer, the farm manager, that the GM crops they were growing - 25 acres of herbicide-resistant oilseed rape developed by the giant German agrochemicals company AgrEvo - would cause "genetic pollution" of the 250 acres of "organically certified" crops on the 1,100-acre farm. Unless the plants were destroyed by 10 June, the farm could not produce "organically certified" food for at least five years.
Yet in March an Association official had personally assured Capt Barker there was no rule against growing both types of crop on the same farm. The new rule was introduced in April by Mr Holden, one of Prince Charles's advisors on GM issues, when he heard about Lushill's GM trial.
Yesterday AgrEvo angrily denounced the Association for "moving the goal posts" and said the new rule, which specifies that organic farmers may not plant GM crops within six miles of organic crops, were "not based on science".
The Lushill crop was planted nine weeks ago and had not flowered. The farm's owners, three trustees - a lawyer, a businessman and a director of a public company - refusedpounds 10,000 the Association offered as compensation.
Capt Barker said: "With great regret I have had to abort my GM trial. I believe very strongly in technology and the bio-tech industry and all the work that has been put into these trials. But the trustees of my family settlement have different views and have all along not been in favour, but recent events have made them come out against this GM trial."
Michael Meacher, the Environment Minister, said he was "sorry and disappointed", and trials were essential to find whether GM crops would harm wildlife and the environment.
Opponents of GM were jubilant. "We are pleased because it is one less plot out there that is threatening to pollute other crops, including organic plots," said Mr Holden.
Doug Parr, of the pressure group Greenpeace, said: "This shows the Government needs to make a choice between organic and genetically modified agriculture." The Association has demanded at least six miles' separation of GM crops, on the basis that bees can carry pollen up to three miles.
It is now examining how close organic farms are to the 147 sites around Britain where GM crops are still being grown.
About 100 square yards of GM oilseed rape plants were ripped up at an experimental site at Tillycorthie Farm, near Ellon, north of Aberdeen on Sunday night. Police are investigating the incident.