Dr Andrew Chesson admitted that some current safety tests could allow harmful substances to enter the human food chain.

Dr Chesson is a senior researcher at the Rowett Institute in Aberdeen, where last year Dr Arpad Pusztai publicised a study suggesting that GM potatoes could damage the immune system of rats.

Then, Dr Chesson sought to reassure the public in the subsequent furore over GM trials. Now, however, he has warned that those tests could be flawed in some cases.

'Better safety' required

In a speech to the Royal Society of Chemistry's annual conference in Edinburgh on Monday, Dr Chesson warned that genetic engineering could create potentially dangerous changes to plant metabolisms, which go unnoticed in existing tests.

Scientists are breeding oilseed rape which produces many useful chemicals

"We will undoubtedly use novel genes that haven't faced the regulatory system before," he said. "There are tens of thousands of potential genes that could potentially be used.

"Thought should be given now to new procedures that will have to be adopted for better safety scrutiny."

Dr Chesson also voiced fears about GM plants grown for industry ending up in the food chain.

Scientists are developing strains of oilseed rape which produce high amounts of useful chemicals, such as jojoba wax for cosmetics. Once the crop has been crushed and drained, it is usually fed to animals.

Friends of the Earth has campaigned hard against GM trials

If there were hazardous chemicals in the feed they could be introduced into the human food chain, said Dr Chesson.

He added: "If we choose to go down that road there could be a number of serious consequences."

Critics of GM trials responded to the scientist's warning by calling for GM ingredients to be taken out of food immediately and a ban imposed on any further releases into the environment.

Friends of the Earth campaigner Adrian Bebb said: "At last a reputable scientist has echoed these concerns which are shared by many consumers.

"This is an extremely welcome admission. These foods are being forced into the food chain without proper tests."