Twenty eight Greenpeace supporters, including its executive director Lord Melchett, have been cleared of causing criminal damage to a field of genetically-modified maize.
All admitted trashing the field in an orchestrated attack at Lyng, Norfolk, in August last year.
But they denied the charges, arguing they acted lawfully to prevent pollen from genetically-modified maize polluting neighbouring organic crops and gardens.
A jury at Norwich Crown Court found them not guilty after a two week trial.
At a trial in April all 28 were also cleared of theft as a result of the same incident but that jury failed to reach a verdict on the criminal damage charge.
Lord Melchett, 52, immediately called on the Government to end the GM farm trials "before any further genetic pollution of the environment occurs".
He said: "Greenpeace wanted to remove the GM maize in Norfolk because we believe that GM crops will inevitably contaminate the environment.
"The Government is currently reviewing separation distances imposed between GM crops and other similar crops - separation distances which we said were completely inadequate when we took action in July 1999.
"GM material is still being used to feed farm animals in Europe although a growing number of retailers - such as Iceland in the UK - are already committed to selling animal products from animals not fed on GM crops. We expect other European retailers to follow suit over the next few weeks."
Lord Melchett told the court he believed the genetic modification of crops was one of the "most frightening things he had ever come across".
He criticised chemical company scientists for "not living in the real world".
The former Labour Government minister also told the court that meetings with Tony Blair had left him feeling that the Prime Minister was determined that GM technology would go ahead.
"I think GM is one of the most serious issues Greenpeace has ever tried to tackle," Melchett, a farmer of Hunstanton, Norfolk, told the first trial, in April.
"GM means putting things into the environment which are alive. You cannot recall it. It is one of the most frightening things I have ever come across.
"We don't have a religious objection to it. We would not object to scientists doing experiments in labs which are contained and controlled.
"What worries me is when you use this for crops you are putting it out into the environment on which all human and animal life depends, and we know very little about it.
"It seems to me extraordinary that scientists would think of putting these artificial GM crops out into the open when they know so little about what they are doing."
He said he was concerned that the public debate on GM crops was too scientific and theoretical.
"As a farmer it seems to me things get spread around in all sorts of ways. Mess gets spread around," he added.
"I feel that a lot of the scientists from the chemical companies who are discussing this don't really understand what it is like on a farm."
Lord Melchett added: "I was very disturbed about the idea that people were going to put this out into the environment having apparently not thought of some very simple straightforward things that only a farmer would be able to tell them.
"I think (chemical company scientists) genuinely believe that what had worked in the laboratory would work outside but I don't think they had much idea, some of them, what the real world is actually like.
"In private I have never met a scientist working for anybody ... who doesn't admit that there is some risk."
Lord Melchett accepted that the risk of something going wrong might be low. But he added: "It does seem to me that if something does go wrong, the consequences would potentially be very, very serious indeed.
"We are dealing with things that are alive. If something goes wrong with something that is alive you cannot stop it. You cannot call it back."
Lord Melchett said he had met a number of Government ministers and had meetings with Mr Blair in late 1998 and early 1999.
"Last summer I had the feeling there was a number of Government ministers who privately had concerns," he added.
"But the Prime Minister was very determined, with some other Government ministers, to make sure that this technology went ahead.
"My experience with two meetings with Tony Blair was that the Government were not going to rein in or slow down the introduction of these crops into the environment."
Melchett said he and his co-defendants had taken the attack on the maize field with a "great deal of seriousness".
"It was a genuine attempt as far I was concerned to stop and remove this genetic pollution," said Lord Melchett, a minister in the departments of environment, industry and in the Northern Ireland office during the late 1970s.
"I went to the field with the intention of trying to remove any crop and trying to return it to the (owner)."
He added: "It had been reported in Farmers Weekly that the crop was about to pollinate within about seven to 10 days.
"It is at the point of pollination that the genetic pollution of a crop of this sort becomes uncontrollable."
The people taking part in the demonstration, who wore white boiler suits but made no attempt to hide their identities, were instructed to act on their consciences and not to be violent, he said.
A leading food retailer told the court that it was almost impossible to guarantee that products were free of genetically-modified material because of pollen contamination.
Malcolm Walker, chairman of the Iceland frozen food chain, was giving evidence in the case of the 28 Greenpeace supporters.
"It is almost biologically impossible for something to be GM free because of cross-contamination or cross-pollination by birds, bees or whatever," he told the first trial, in April this year.
"The public do not want contamination of any degree."
Mr Walker, a member of Greenpeace for 10 years, said that when Iceland launched a range of food it was careful to label the products "not made with GM materials".
"We were very careful never to say the products were GM free," he said. "It might seem to be splitting hairs but the issue was contamination."
Greenpeace also said it was delighted with the verdict, which it hailed as a "landmark".
A spokesman added: ""We always said we were acting to protect other crops and the jury clearly believed us."
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