A seed company could face prosecution after unauthorised genetically modified seed was found in crop trials in Oxfordshire.

An inquiry was launched after seed company Aventis told the Department of the Environment that two small GM sugar beet trial sites contained a tiny amount of unauthorised seed.

The sites with the 0.5 per cent of unauthorised GM beet seed were in Abingdon and at Shelford, Cambridgeshire, which were among ten experimental sites planted in England this spring.

One site suspected of being used for the trials was Hill Nurseries at Appleton, near Abingdon.

The matter was referred to the Advisory Committee on Releases in the Environment.

It said the sites would have to be monitored for two years but there was no threat to human health or the environment.

Government-appointed experts were expected to make an initial report this week.

Jean Saunders, of Friends of the Earth Oxfordshire, said: "We don't yet know the effect this has on humans so I don't know how they can be so confident that there is no risk."

No-one at Hill Nurseries was available for comment.

The news came as it was revealed a tennis-court sized field in Oxfordshire is among five areas being used for secret trials of GM crops, about which the Government has failed to release details.