An investigation has been announced by farming minister Elliot Morley after rogue seeds were found to have contaminated a trial of genetically modified rapeseed crops.

But he refused requests from environmental groups for an end to the tests.

Material was found during a routine check in fields on the Aberdeenshire plot, one of two in Scotland. There are also 12 sites in England.

"We have to have these trials to see if there are problems that can be identified," Morley told the BBC Radio 4 Today programme.

"Clearly a problem has been identified. We will be looking at the full procedures which includes the role of the GM inspectorate and whether we need to strengthen that as well."

Speaking on the same programme, the director of the Soil Association, Patrick Holden, claimed that the trials were "an accident waiting to happen".

His sentiments were echoed by Friends of the Earth. "This is yet another biotech blunder from the GM industry. How can we trust them to produce our food if they cannot even run a GM test site," said the group's GM campaigner, Adrian Bebb.

"This is the latest in a number of GM incidents involving Aventis. It is clear that they are incompetent and should not be allowed to experiment with our countryside or our food any more

"The government must listen to its own advisors and pull the plug on the proposed winter trials until they know how this contamination took place. It beggars belief that their own Inspectorate visited Aventis in April but did not uncover this contamination.

"If Aventis has breached its consent then the government should use the full force of the law."

Liberal Democrat environment spokesman Malcolm Bruce warned that stricter scrutiny of trials were needed.

"This incident highlights the need for strict monitoring of GM trials. The challenge for companies involved in GM is to inspire confidence in the public and it is news like this that will undermine public feeling," he said.

"If companies are unable to manage test crops then it does not bode well. Further testing is needed and should take a very cautious approach."

However, biotechnology groups described the finding as "a technical breach".

Aventis CropScience, the company who supplied the seeds, insisted that there were no implications for health.

"This is not a safety issue since GM oilseed rape varieties containing these events have been grown in trials in the UK since 1989, and commercially in Canada since 1995 on many millions of acres, without one reported detrimental incident regarding human and animal health or the environment," said a spokesman.