There were originally seven trials running, but two were disrupted by environmental activists, and another was discontinued after a farmer withdrew from the scheme.

A group of 28 protesters, including Greenpeace chief Lord Melchett, were charged after an alleged attack on a trial farm in Norfolk on Monday. All have now been released on bail.

But the worry for scientists running the trials is that the remaining sites could also be destroyed. Protesters have said GM crops are contaminating the environment and are legitimate targets.

Police are stepping up security around the crops

The locations of farms are common knowledge among anti-GM activists. An Internet Website - The X Fields - details their addresses as well as support for Greenpeace's cause.

Scientists fear that test fields in Redbourn, Hertfordshire, could be the next target.

Roger Turner is chairman of the Supply Chain Initiative on Modified Agricultural Crops (SCIMAC), a consortium for the agriculture industry. He is concerned for the welfare of the existing sites.

"The farms need extra protection," he told BBC News Online.

"Police security in the test areas is on a much greater state of alert and vigilance, and there are now police cars stationed there."

He stressed the importance of the trials, saying the UK "led the way" in Europe for GM research.

Scientists admit it is impossible to make an open field totally secure

"There is not a lot of similar work being done in Europe," he said. "The government and industry listened to the concerns of those against GM crops, and set up the tests.

"Now protesters want to destroy them - they are acting undemocratically. It is very sad and lamentable".

As to whether the tests will suffer, with almost half their crops destroyed, scientists appear to be divided.

A spokesman for campaign group the Genetic Engineering Network said on Tuesday that the whole experiment in Norfolk had been "completely invalidated by the destruction carried out by activists".

Results 'not affected'

But one of the scientists assessing the trial results said "all is not lost", and that even the destroyed sites could still render information.

Dr Les Firbank, of the Institute for Terrestrial Ecology, in Cumbria, told BBC News Online: "Of course we will now rely more heavily on the remaining sites, but we can still monitor the sites targeted by protesters.

"In fact, we'd already had a lot of data from the Oxford site that was damaged last week."

Dr Firbank said the main job of the test sites was to learn the "methodology" for how best to assess GM crops in the future, as well as studying biodiversity.

"They're about what and when to measure," he said. "This year's results won't affect the overall results anyway."

His main concern was for the farmers, who he said had suffered "intimidation from protesters, which is totally unacceptable". He would not reveal the farmers' identities, saying they were already under enough pressure.