Government scientists are investigating possible contamination of two conventional varieties of oilseed rape by modified genes. Both varieties were undergoing field trials at research sites when scientists discovered that they contained DNA commonly used in GM plants.

Scientists from the Central Science Laboratory, the official GM inspectorate, are carrying out further tests to try to determine how the contamination occurred.

One of the plants, a spring-sown variety called Dorothy, had been imported from Germany and had been planted on a small trial plot, the Ministry of Agriculture, Fisheries and Food said yesterday. "These plants will be killed by spraying with herbicide as soon as they germinate," Maff said.

The second plant, a hybrid winter variety which is too new to have been given a name, was undergoing trials for being included on the national list of approved crops. It was sown last autumn in test and trial plots, Maff said.

"Trial managers have been instructed to mow affected plots immediately to avoid cross pollination with adjacent fields," said Maff.

The Dorothy variety tested positive for the presence of a genetic promoter called P-Nos, which is commonly used to transfer genes into GM plants. The promoter also occurs naturally in soil bacteria that are also used to produce GM crops, a spokeswoman for Maff said.

Adrian Bebb, a spokesman for Friends of the Earth, described the discovery as worrying. "The big question is, how did it get there? It just highlights that we can't control GM crops.

"We are not going to stop the wind blowing pollen and the bees carrying it," he said. "If we are going to grow GM crops then they will contaminate seed."

The more GM crops that the Government allowed to be grown, the higher the risk, he said. "Cross-contamination is going to go on and on. The two means of farming just don't go together.

"Consumers are going to have no choice eventually. It is extremely new technology and we don't know enough yet about all the safety implications. If it is out there and already getting into the food chain, then that is really worrying," he said.