A senior Monsanto director has also conceded that insects are capable of developing a resistance to plants genetically engineered to kill them, and that "resistance" in plants and insects from GM crops is "a very real thing".
Gary Barton, director of biotechnology communications for Monsanto in the United States, told the Independent on Sunday that "resistance can develop" but that "superweeds" - hybrid plants resistant to insecticide - were not a problem since they could always be sprayed with other weedkillers to which they were not resistant.
UK environmentalists and the Government's own wildlife advisers have seized on the admission as "groundbreaking" because they claim it proves Monsanto has known all along about the dangers of genetically engineering plants.
The admission will be raised this week by members of the Commons Environmental Audit Committee, which is to question Jack Cunningham, chairman of the Cabinet committee on biotechnology. Dr Cunningham told the Commons last week that experience showed cross-pollination was "not really a very significant problem".
Pete Riley, of Friends of the Earth, said: "It appears that Monsanto is happy for Europe to come under threat from GM crops, while taking a completely different attitude on their own doorstep."
Monsanto's contracts for US farmers contain detailed "Resistance Management" plans, including planting blocks of non-GM crops around fields. InBritain, the company has applied to market genetically-modified sugar beet, which, according to scientists, would be capable of breeding with non-GM cousins. "If Monsanto is concerned about cross-pollination in the States, they haven't shown much sign of concern here," said Brian Johnson, senior adviser to English Nature.
Last week, the Independent on Sunday revealed that scientists claimed to have found the first GM "superweeds" in Cambridge, after GM oilseed rape plants bred with wild turnips.
Mr Barton said that the spread of pollen "is an issue that has to be addressed". But he added that a hybrid plant that was resistant to "roundup ready" herbicide was not a superweed because it could be killed by another weed killer. "All that stuff about superweeds doesn't mean anything. It means it is resistant to Roundup Ready [a herbicide]. It's not a superweed if it becomes resistant to Roundup - you can use another weedkiller," he said.