The federal government had forbidden a manufacturer to start commercial production of six possibly toxic new industrial chemicals, raising a question whether similar substances already in widespread use may pose a serious risk to human health and environment.

The order -- the first of its kind to be issued under the 1976 Toxic Substances Control Act -- was announced yesterday by the Environmental Protection Agency.

"We cannot afford any more incidents . . . where we find out too late that commonly used, but insufficiently tested, chemicals have caused grave human or environmental damage," EPA Deputy Administrator Barbara Blum told a press conference. As examples of such incidents, she cited the discharge of Kepone into Virginia's James River and of chemicals known as PCBs into waterways throughout the country.

Under the law, EPA yesterday could not identify the company involved, nor say how much of the planned chemicals it had planned to produce.

The six new chemicals are added to polyvinyl chloride plastic products, such as garden hoses, floor tiles, and refrigerator gaskets, to make them flexible.

Called phthalate esters, the chemicals are members of a family known as plasticizers. Last year, more than 1.5 billion pounds of plasticizers were produced.

EPA's order was triggered in part by a previously undisclosed two-year study by the National Cancer Institute of DEHP, a plasticizer closely related to the six new chemicals. More than 500 million pounds of DEHP -- percent of total plasticizer production -- were made in 1979.

According to the order, the study "strongly indicates" that DEHP caused liver cancer in rats and mice fed various doses of the substance. The similar new chemicals "also may be carcinogenic," the order says.

The order termed it "highly significant" that liver malignancies developed in none of the untreated "control" rodents, but did develop at high rates in two strains of treated animals: in five of 50 male and eitht of 50 female rats, and in 24 of 100 female mice.

The unpublished cancer institute report, dated last March 5, was not known to the unidentified manufacturer, EPA Assistant Administrator Steven Jellinek told reporters.

Plasticizers related to the new chemicals also have been shown to kill or cause skeletal deformities in rainbow trout, brook trout, and other fish, and to impair reproduction in lower waterborne organisms in the food chain, Blum said.

For that reason, the agency asked the company to submit test results or other information on a possible cause-effect relationship between the new chemicals and cancer, and also on their possible potential to kill fish and to concentrate in fish eaten by humans.

In premanufacturing notifications required by the law, the company reported no safety studies at all and is assumed by the agency to have done none. The EPA acted after reviewing studies on existing plasticizers.

The company is under no time limit to submit the requested information. But, Blum said, it is prohibited from starting to manufacture the new chemicals unless it satisfies the agency that the substances can be made, used and disposed of in ways that will avoid serious harm to people and the environment.

Blum expressed hope that the company will opt for "cooperation" that "will set the tone for future cases," but it could fight the order all the way to the Supreme Court.

The agency is empowered to block commercial production of a new chemical, within 135 days after being notified of an intent to produce it, if it finds a significant risk.Over the objections of the nonprofit Environmental Defense Fund, the EPA has allowed production of about 30 new chemicals for which no safety data was provided, on the ground that it knew of no significant hazard.

As for the existing plasticizers, Jellinek said that EPA intends in the next year or so to propose that they be tested for health and safety.Because of the focus of the law on the introduction of possibly unsafe new chemicals and because of limited EPA resources, removal from the market of the existing plasticizers would be difficult, he said.

If the new chemicals were to be produced, the EPA estimated 300 to 400 production workers and 1,000 to 10,000 plastics-goods workers would be exposed to them. Jellinek said that there is little evidence involving a possible hazard to consumers, but that EPA does have "a concern."