"The plants themselves become weeds in the next crop - it's very difficult to kill the oilseed rape plants when they come up the next year and of course they come up in another crop like corn," English Nature's biotechnology advisor Brian Johnson told Reuters.

"The last thing farmers want is oilseed rape plants in amongst the corn. It's becoming an agricultural problem and farmers are having to use older herbicides to control them - those herbicides are less environmentally friendly and more toxic," he added.

Johnson said the government should limit the number of herbicide-resistant varieties to prevent the problem of super resistant plants arising in Britain if commercial growing of GM crops is approved.

"What we're saying is that the limit might be one variety instead of three as seen in Canada," he said.

He also urged a re-think of the European Commission's recent proposal for a one percent threshold of GM seed in batches of conventional seed.

"We're saying this is a bad idea because if you do that, that one percent could be seeds made of three different GM varieties," he said.

"In agreeing threshholds for those seeds, the European Commission may well be producing exactly the type of problem we're seeing in Canada."