The research into developing the plants came under fire after its publication in a scientific journal, when it was revealed that the prime mover behind the technology was Monsanto, the American biotechnology giant.
Pete Riley, a campaigner for the environmental group Friends of the Earth, called it "a public relations stunt using what is perceived as a beneficial use of GM to repair [Monsanto's] damaged image".
But scientists at the company said they had been working on the plants since 1994, long before the present storm over GM crops blew up.
The new GM plants, principally oil seed rape, contain four added genes from bacteria that naturally produce a biodegradable plastic called PHBV. Inserting the genes and manipulating the plants' metabolism was "a considerable feat of genetic engineering" according to Nature Biotechnology magazine, which has published a peer-reviewed paper on the experiment in its latest issue.
"The bottom line is that it works," said Ken Gruys of Monsanto, the lead scientist in the research. "This is really a first step. The plants aren't producing a large volume of plastic, but it is on the way."
Using bacteria alone to produce plastic costs five times as much as producing it from crude oil, the principal source of plastics and petroleums. But the price of extracted oil and its derivatives is expected to rise, which would make GM plants more competitive as a source. Furthermore, fossil- based plastics decompose slowly, leading to waste management problems.
Dr Gruys said: "This is a very important first step but I think we would be fortunate if we saw this in [commercial use] 10 years from now."
The sugar industry in both Australia and Brazil aims to use GM and conventional technologies to produce biodegradable plastics from sugar cane.
However, Mr Riley saidpeople should not be using so much plastic in the first place. "If we're worried about plastic we need to deal with the human end." Alternatives, such as paper wrapping, should be examined. "We've already got a natural plastic. It's called cellulose," he said.