Margaret Gilmore:"Government scientists have surveyed the damage"

The incident leaves just four of the original seven farm trials of GM crops intact - two have now been disrupted by environmental activists and another was discontinued after a farmer withdrew from the scheme.

BBC Environment Correspondent Margaret Gilmore says the future of the whole programme is now in doubt.

In the latest incident, Greenpeace protesters broke into Walnut Tree Farm, a government-sponsored GM trial farm in Lyng, near Norwich, on Monday morning.

They used a mowing machine and strimmers to rip up as much of the six-acre field of maize as they could.

Greenpeace used a mower to cut down the crops

Workers then tried to drive them away with a mechanical digger, and farm manager William Brigham, 59, disabled the cutter by ramming it with his tractor.

However, the protesters continued trampling the crop by foot for a further ten minutes until police arrived.

30 arrests

Police arrested 30 activists, including Greenpeace executive director Lord Melchett, who has an organic farm in Norfolk.

They were taken to various police stations throughout the county. Twenty-eight were charged in connection with the destruction of the maize crop.

The BBC's Andrew Sinclair: "It was a very organised protest"

Later in the day, the brother of Mr Brigham collapsed in the field and was taken to Norfolk and Norwich Hospital where he was treated and then released.

Mr Brigham, an outspoken supporter of GM testing, accused Greenpeace of "anarchy".

"I was woken to find about 40 people on the site with a tractor with a cutter on the back trashing the trial," he said.

"This has nothing to do with genetically-modified organisms - it's whether we want democratic government in this country or anarchy."

'Vandalism'

Agrochemical firm AgrEvo, which had planted the crops, was similarly infuriated by what it termed Greenpeace's act of "vandalism".

"We utterly condemn this deliberate act of trespass and criminal damage upon private property," AgrEvo said in a statement.

About half of the six-acre site was destroyed

"The government has to decide whether it or Greenpeace and other pressure groups run this country."

AgrEvo spokesman Des D'Souza called for the addresses of farm-scale trials, which by law have to be made publicly available and are published in newspapers and on the Internet, to be kept secret from now on.

He said his company had been "open and transparent" in declaring where the field scale trial sites were.

"The result of it is it now allows these people to come in on their own agendas and cause this damage," he said.

Sites 'may be kept secret'

The government conceded that GM trial locations may have to be kept secret in future, but said GM testing will continue despite the Greenpeace attack.

The Minister for the Cabinet Office, Dr Jack Cunningham, said Greenpeace's attitude was "absurd".

"How can we possibly have an informed and rational debate about protecting the environment when there are people destroying the very evidence we need to conduct this debate?" he said.

"All responsible groups should disassociate themselves from reckless criminal damage of people's property," he said.

"We intend to continue field and farm scale trials exactly because we want to ensure that any decisions we take are based on the best scientific evidence."

Attack 'in public interest'

But Greenpeace insisted that the protest had been a peaceful action, on behalf of the British public, against what it described as "genetically-modified pollution".

Lord Melchett said: "The British public have made it very clear that they do not want these GM farm-scale trials to go ahead.

"Now that three out of seven of the government farm-scale trials have been disrupted, the whole programme of commercialisation of GM pollution disguised as science is at risk."