WASHINGTON -- Makers of genetically modified crops have avoided answering questions and submitted erroneous data on federal applications aimed at ensuring the safety of bioengineered foods before they are marketed, a study by a Washington advocacy group shows.

The Center for Science in the Public Interest, which sees benefits in biotechnology crops but wants tougher regulation to ensure safety, said its study of 14 applications to the Food and Drug Administration over the past seven years points out significant holes in the safety review of new biotech crops.

A flawed review process could deal a huge blow to efforts to market genetically modified crops. Proponents and opponents alike see the reviews as a crucial way to build consumer confidence -- especially amid strong resistance in Europe and other export markets.

Under current FDA rules, if genetically modified crops are shown to be "substantially equivalent" to conventional crops, further government review isn't needed before the crops can be marketed. The company is responsible for doing the safety review and showing its results to the FDA. Of the 14 submissions obtained by the CSPI under the Freedom of Information Act, the FDA requested more information in six cases. In three cases, the companies provided the extra data but the other three told the agency it wasn't needed. The FDA has no legal authority to compel the companies to submit the data.

When Monsanto Co. applied to market two varieties of insect-resistant corn, it included data to show that the nutritional content of both genetically modified grains wasn't affected. But for one of the varieties, Monsanto didn't submit nutrient data for the rest of the corn plant, such as the stalks, which often go into animal feed. When the FDA sought that data, it was rebuffed. A Monsanto spokesman said the company would have addressed the FDA's concerns if it hadn't decided against marketing the product. The CSPI also said the FDA overlooked factual and scientific errors in applications for genetically engineered tomatoes and cantaloupes. The developer, Agritope Inc., which has since been acquired by Exelixis Plant Sciences Inc., claimed the tomatoes and cantaloupes posed little risk because humans were already naturally exposed to the protein they were engineered to make. But the scientific papers submitted to prove this point didn't say that. A company spokeswoman said the FDA never mentioned the gap in the data, and the project was dropped before marketing.

The FDA defended its review process, saying that all biotech products on the market were safe. "In every case where we thought more information was critical to the company's safety assessment or our review, the companies have sent the information and done additional studies," said Laura Tarantino, director of the office of food-additive safety at the FDA. The agency has always been open to changing its review process if new biotech products demand greater scrutiny, Dr. Tarantino said.

In general, an FDA review is limited to a "voluntary consultation" in which it examines a company's scientific information on potential allergic and toxic reactions, and the nutrient content of the foods. In theory, if the FDA found that a genetically modified product wasn't "substantially equivalent" to a conventional crop, it could require a more rigorous review as a food additive. But that has happened only once in the 53 voluntary consultations conducted between 1994 and 2000.

The biotechnology industry has long said it treats the FDA premarket reviews as if they were mandatory. The Biotechnology Industry Organization, the industry's trade association, known as BIO, also supports proposed regulations to make the voluntary consultation mandatory. But critics say this wouldn't change the fact that companies determine the content and depth of safety reviews and not the FDA.

"I find it highly unlikely that companies have not provided all information requested to FDA because every company wants the FDA letter of safety in order to market their products," said Lisa Dry, a BIO spokeswoman. "The FDA has all the necessary authority they need to protect the public and make sure foods are safe."

CSPI Science Director Doug Gurian-Sherman said the review found that "biotech companies weren't always performing the right tests to look for potentially dangerous compounds, including allergens, and that there was a great deal of unevenness among different developers' submissions." Since the FDA doesn't tell the companies what to test for, analysis of some known toxins and allergens was left out of some applications.

While most biotechnology experts agree that the crops now on the market are safe, some are concerned that current regulations are too weak to ensure that new products will be safe, too. "The key question is whether the voluntary notification system is up to the challenge of evaluating the products in the pipeline," said Mike Taylor, former deputy commissioner for policy at the FDA and adviser to Monsanto. New products -- rice enriched with vitamin A or tomatoes engineered to boost cancer-fighting antioxidants -- are engineered to produce desirable proteins and could present novel questions about allergic and toxic reactions.

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