Professor Alan Gray, a member of the Advisory Committee on Releases to the Environment, said that crops engineered to be resistant to attacking viruses could eventually lead to "superweeds" through the transfer of the resistance genes to wild plants.
His views reflect the gulf between the regulations in the US and Europe - but world trade rules could lead the US to insist that such crops are approved in Britain, despite scientific opposition.
The US approval covers a transgenic squash, a melon-like plant, developed by Upjohn Agrochemicals. It contains genes making it resistant to a virus that infects watermelons and to another that attacks courgettes. It was passed for sale in 1994 because US government scientists assume that plant viruses, which attack leaves and roots, are particular to plant species and not weeds.
Such plants are a "second-generation" form of GM crop. Most of the "first generation" are resistant to artificial fertilisers rather than natural viruses.
But Professor Gray, based at the Institute of Terrestrial Ecology, said that ecologists still knew too little about the effects of viruses on plants.
"Ecologists have neglected viruses because they're so hard to find and detect in plants. I don't think a UK regulatory committee would release a virus-resistant plant unless we really knew about the role of the virus," he said.