Sugar-beet plants near Fakenham have been ripped up, from what scientists say is a small-scale test site.

The plant had been modified to be herbicide-tolerant, and was being grown to assess its environmental impact.

The research is being carried out by independent scientists from the Institute of Arable Crops Research.

The BBC's Alex Dunlop speaking to Robert Hambridge, Farmer.

The trials are providing "vital research", according to Dr John Pigdeon from the institute. He said the results could answer some of the questions in the current debate about GM crops.

Ten square metres of the 2.5 acre-field were damaged

But Dr Pidgeon said the implication from the crop protests is that there are "some people who want to stop the study and deny the public important information on the issue".

BBC's Linda Stewart: "About 50 similar small-scale trials are taking place, with some reportedly suffering similar attacks"

Farmer Robert Hambridge, who owns the field, told the BBC that protesters had "spoiled a very useful trial".

"It wasn't set up for commercial purposes - it was for the environment, to see the effect of GM crops," he said.

Mr Hambridge said pollen from flowering sugar-beet plants would not have been a problem, as it flowers bi-annually, and is not due to flower this year.

Environmental protesters have expressed concern that GM pollen will spread to nearby crops and contaminate them.

Mr Hambridge also expressed concern that addresses of farms taking part in the trials are available on the Internet, but said they would be continuing.

Previous crop damage

There are about 50 small-scale trials taking place around East Anglia, and the scientist says some of them have suffered similar attacks.

Earlier this week, 28 Greenpeace demonstrators were charged after GM maize was damaged at Lyng, also in Norfolk.

But Greenpeace chief Lord Melchett, who was among those arrested, said he did not regret being arrested and held in custody over his protest. He also said the pressure group had nothing to do with the crops destroyed on Wednesday.

Lord Melchett was led away by police

He said he believed a lot of people supported his action in trying to stop "a very serious threat to the environment".

The second attack highlights scientists' concerns that the remaining trial sites will be targeted by protesters, threatening the outcome of the test results. Security at four of the original test sites was considerably increased earlier in this week.

So far the data has not been affected, and Dr Les Firbank, of the Institute for Terrestrial Ecology, in Cumbria, told BBC News Online that data from the damaged sites could still be collected.