CPB Twyford said protesters had caused thousands of pounds of damage to its crops.

John Blackman, the Cambridgeshire firm's technical director, said a number of test sites across the country had been damaged in recent months.

The move, thought to be the first by a seed company since the controversy about GM crops emerged, was welcomed by environmental campaigners.

Mr Blackman said vandals had not only attacked trials of GM crops, but also fields containing tests of non-GM crops.

He said: "A number of fields have been damaged by people who presumably believe they are damaging GM crops.

Environmentalists fear GM crops could damage wildlife

"But in only one case has the field actually contained a GM crop. In all the other cases they have been trials involving crops that are not genetically modified.

"It seems that people are assuming that all trial sites contain GM crops or they are destroying plants just in case they are modified."

He added: "It was felt that the risks involved in continuing with work on GM crops were not worth taking while the threat of indiscriminate vandalism still exists."

'Mob rule'

A spokeswoman for the Genetic Engineering Network, which passes on information to opponents of GM crops, welcomed CPB's move.

"I think the move probably takes account of public opinion about GM crops - not just direct action," she said.

"This is very good news. Hopefully some of the bigger companies will now make the same decision and stop their trials."

But the British Association of Plant Breeders said "mob rule" could not be allowed to triumph.

'Moral obligation'

"We cannot have scientific progress and its acceptability determined by lawlessness and mob rule," said a spokesman.

"The benefits of otherwise of these crops must be properly monitored and evaluated."

The green lobby in the UK wants a four-year moratorium on the licensing of GM crops. The government has refused, insisting science should judge how long it takes for the technology to prove its safety - not some arbitrary time period.

The government agrees that we need to know more about GM crops and believes more, and larger, field trials will be necessary to test the issues. Gaining good scientific data through trials has to the sensible way forward.

On Thursday, the influential Nuffield Council on Bioethics said there was a moral obligation to develop GM crops.

Ban rejected

A report produced by the think-tank says the new technology could bring benefits to developing nations by helping to feed growing populations.

It also said there were no grounds for a ban on GM food or moratoria on commercial planting, which environmental groups have been demanding.

But it also warned that better regulation is needed to ensure GM crops do not damage human health or the environment.