Sodium nitrite, a chemical additive used in most processed meats, causes cancer in laboratory animals, according to a study conducted for the Food and Drug Administration.

The results of the study, conducted by Dr. Paul Newberne of the Massachusetts Institute of Technology, means the FDA must consider banning nitrites, which are used in over two-thirds of the pork and 10 percent of the beef produced in the country.

Under the 1958 Delaney amendment to the Food, Drug and Cosmetic Act, any substance that causes cancer in man or animal when ingested must be banned.

The FDA and the Agriculture Department, which has regulatory authority over beef and pork, said in a joint statement last night that they would assess "several options" because they wanted to "weigh the risks associated with nitrite added to food against the health risk from not adding it.

Until this study, it had been throught that sodium nitrite was a threat only when combined with amines in the meat and in the human stomach to form nitrosamines, which are known to cause cancer.

But now sodium nitrite alone has been identified as a chemical carcinogen.

Scientists have argued for almost 10 years over whether nitrites are necessary to prevent the formation of the deadly botulinum toxins, or are used just to give the characteristic color and flavor to processed meats such as bacon, ham, hot dogs, liverwurst, bologna, salami as well as chicken hot dogs, and some smoked fish like lox.

A number of small manufacturers have been producing nitrite-free products on a limited scale. They rely on other methods to prevent the toxins from forming, including freezing, refrigeration for a limited time, and smoking.

These methods require more care in processing transportation and storage, and if adopted by the large meat processors would probably add to the cost of the products.

Before a ban could go into effect consumers would have to be educated on the handling of these nitrite-free products because they would spoil more rapidly.

Assistant Agriculture Secretary Carol Foreman said recently that if nitrites were banned, "You won't be able to leave uncooked bacon out all night and you won't be able to take uncooked hot dogs to the beach all day and then warm them slightly on a fire and eat them."

Several consumer groups have more than once petitioned the government to ban the use of nitrites in food, but always on the grounds that they combine with amines, found in drugs, tobacco with amines, found in drugs, to form the carcinogenic nitrosamines.

But in Newberne's study 13 percent of the rats fed nitrites developed tumors of the lymph system, while 8 percent of the rats not fed nitrites developed tumors. The government's statement called the difference "statisfically significant."

A spokesman for the meat industry declined comment on the announcement because he had not seen the study.

But consumer activits who have been active in efforts to ban sodium nitrite say the government is not doing its job.

Ralph Nader said that "Carol Foreman and Donald Kennedy [head of the FDA] should go back to the law and read it because they are violating it."

Anita Johnson, a lawyer with the Environmental Defense Fund, accused the agencies of "laying low when they should be racing ahead."

Health, Education and Welfare Secretary Joseph A. Califano Jr. has had the FDA evaluation of the study on his desk for a month. Sources said that he had not allowed the results to be released earlier in order to plan the best strategy for avoiding a confrontation with Congress similar to the one that developed when FDA announced its intention to ban saccharin. At that time Congress passed a law placing an 18-month moratorium on the saccharin ban until further studies were completed.

The same sources said Califano was responsible for the cancelation of a joint USDA-FDA press conference, which had been scheduled for today. The joint statement, issued last night, was in response to press inquiries.