The Agriculture Department and Food and Drug Administration intend to start removing the suspect preservative, sodium nitrite, from the nation's meats next year, it was learned yesterday.

They are moving in that direction as a "high priority," sources in both agencies said, because of a finding that the chemical causes so many cancers in rats that it is assumed it must cause human cancer, too.

The FDA and USDA have developed a 49-page "action" plan - a document obtained by The Washington Post yesterday - and asked the Justice Department to review its legality.

The plan calls for phased removal of nitrite over "several years" to give the food industry time to develop other ways to preserve cured meats, fish and poultry products to prevent deadly botulism, which sometimes occurs as a result of bacterial growth.

If the action plan is approved by the Justice Department, USDA and FDA officials expect to publish a detailed proposal in Federal Register this fall. They would then allow three or four months for public, scientific and industry comment, then some months longer to develop final regulations.

"If Justice approves, that's the way we intend to go," Carol Tucker Foreman, assistant agriculture secretary for food and consumer services, said in an interview. "I don't really anticipate any other trouble."

Another officials said, "We have not yet made a final decision. But a gradual phase-out seems to be the most sensible course to prevent any risk of cancer, and yet no undue risk of botulism in the meantime."

The phase-out would nonetheless mean drastic changes in the way the country's food processors, food sellers and consumers store and use about $12.5 billion a year worth of processed meats, including more than two-thirds of all pork and a 10th of all beef.

Nitrite is used as a preservative in bacon, ham, hot dogs, sausages and luncheon meats, smoked fish, some cheeses and in new poultry products like chicken hot dogs and "turkey ham."

Scientists for years have been concerned about sodium nitrite's possible role in causing cancer. But they were most worried about nitrite's combining with chemicals called amines - both in meat and in the stomach - to form cancer-causing substances called nitrosamines.

The FDA, however, had commissioned what it calls "a major study" of the effect of nitrite alone by Dr. Paul Newberne of the Massachusetts Institute of Technology. It was disclosed last week that Newberne has found that 12.5 percent of rate fed soliom nitrite developed lymphomas - cancers of the lymph system - and another 11.4 percent developed pre-cancerous conditions.

The USDA-FDA action plan reveals that many of the rats also suffered severe damage to their immune system, which combats most disease and infection.

Under the plan, the USDA and FDA - which have sometimes overlapping jurisdictions on various products under various laws would almost immediately ban use of sodium nitrite merely to preserve food color rather than to prevent spoilage.

Longer periods would then be allowed to let the food industry develop other methods of food handling and preservation - such as smoking, freezing, refrigeration, heat sterilization and salting - and to teach consumers how to use nitrite-free foods without peril.

The American Meat Institute signaled the start of a fight against any far-reaching ban by calling the MIT study "inconclusive." Rep. William Wampler (R-Va.), ranking Republican on the House Agriculture Committee, and Rep. James Martin (R-N.C.) introduced both a bill and a House resolution directing the secretary of agriculture to delay any action until three months after completion next year of a study of scientific ability to assess cancer causation - a study inpired by the concern over whether saccharin causes cancer.

Agriculture's Foreman and FDA Commissioner Donald Kennedy late yesterday issued a joint statement saying, "When a regulatory decision has been selected from the options being considered, a full description of our plans to carry it out will be presented for public and congressional criticism."