The human food supply is in danger of being contaminated by crops genetically modified to create better drugs and industrial chemicals, a group of veteran scientists and academics is warning.

The warning is in a strongly worded letter by four PhDs — among them the former dean of science at McMaster University in Hamilton — who advocate mandatory food labelling and better testing of genetically modified foods.

The letter, obtained by The Globe and Mail, says there is a "high probability" the food we eat could be contaminated as a result of sloppy farming practices and the "arrogance" of biotechnology researchers and regulators.

Genetically modified foods have sneaked up on Canadian consumers, many of whom don't know plants that engineered with foreign genes to be resistant to pesticides or herbicides have been researched, grown and consumed here for years.

The letter specifically warns that the pollen of modified plants can transfer engineered genes to unmodified plants growing in nearby fields and that modified traits can spread by "spillage of seed or dispersion of seed by the wind."

Such questions have long been raised about genetically modified conventional crops. Research into molecular farming — the practice of designing plants that grow proteins used to make plastics or medicines — has added to the fears.

The researchers call Canada's introduction of genetically modified food insidious and argue that the only crops that should be used in molecular-farming experiments are those not consumed by humans or animals.

Already, some molecular-farming projects are considered risky enough to be held in mine shafts or under glass covers to protect against the spread of seeds and pollen.

The letter — signed by retired Agriculture Canada scientist Bert Christie, former McMaster University science dean Dennis McCalla, McGill University animal-science professor Dick Beames, and Hugh Lehman, an expert in agricultural ethics at the University of Guelph — is a submission to the federally appointed Canadian Biotechnology Advisory Committee, which is gathering feedback on genetically modified foods as it prepares to advise federal cabinet ministers.

Some of the letter's authors have previously written essays for a group called Genetic Engineering Alert, whose Web site is maintained by the Council of Canadians. Mr. Christie says his name was put forward for a seat on the advisory committee, but he did not join it.

The submission will likely further fuel the debate over genetically modified foods. Earlier this year, a Royal Society panel of experts looked at the issue and argued that Canada's food-safety system is plagued by conflicts of interest, a lack of transparency and ambiguous testing.

The four PhDs make frequent reference to the findings of the Royal Society, a national body of distinguished Canadian scientists and scholars, and criticize the advisory committee both for favouring the views of industry and underplaying the panel's importance.

Peter Phillips, co-chairman of the advisory committee's genetically modified food group, said the Royal Society's report is a part of wide-ranging feedback it is gathering and that a few members of the expert panel now are part of his committee.

He said market forces may help ensure rigorous testing continues on plants modified to produce the protein building blocks of drugs or industrial materials.

"Anybody that does that [research] is going to want to contain that stuff anyway," he said. "They're going to bear the liability if they fail. Nobody is going to want a product in the market that's going to hurt anybody."

Most genetically modified crops so far have been bred to be pesticide- or herbicide-resistant. Worldwide, a number of experiments are under way, including ones that involve adding a strand of human DNA into alfalfa plants, causing canola plants to produce plastic-making polymers, and trying to make a blood protein grow in rubber plants.

As with all matters pertaining to genetically modified foods, no one disputes that safeguards are needed: The question is whether emerging and existing regulations are adequate, and whether genetically modified crops are inherently more risky than traditional crops.

"The reality is the food system has a lot of risk now," Mr. Phillips said. "Some of the new technologies may be less risky than the existing technologies; some may be more."

Genetically modified crops, which some critics denounce as "frankenfoods," are seen by proponents as profitable, a natural evolution of farm science that could help feed a hungry world.

There have not yet been any health disasters stemming from altered crops, but modified corn intended for animal feed has ended up in the human food supply in the United States, and a Brazil nut gene was transferred to a soya bean, bringing with it an allergen.

In Alberta, three different strains of herbicide-tolerant canola grew in close proximity to one another, creating triple-tolerant canola. The fear is that sloppy agricultural practices could result in the resistance being passed to weeds, creating superweeds.

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