Federal investigators say they have evidence that the nation's biggest commercial testing laboratory deliberately falsified data submitted to the government on potential carcinogens and that at least four major pesticide manufacturers were aware of the problem.

The laboratory, Industrial Bio-Test of Northbrook, Ill., has been under investigation by probers from the Environmental Protection Agency and the Food and Drug Administration since last fall. Both EPA and FDA have referred the results of their investigations to the Justice Department.

Knowledgeable federal regulatory officials said yesterday that audits performed on the commercial lab by EPA and FDA have turned up far more problems than investigators had originally expected.

"At the beginning of this thing we thought we were dealing with problems confined to just one three-year period," said a senior official involved in the probe. "Now we are in a position where we have to regard all of IBT's data as suspect."

The extent of the problem caused by the alleged data tampering by the lab was noted by the official, who said that the World Health Organization, which sets international safety exposure standards for potentially harmful chemicals and pesticides, also relied heavily on data from IBT.

In addition to alleged data irregualarities at IBT's Northbrook lab, investigators say they are also questioning tests by IBT labs in Decatur, III., and Neillsville, Wis.

Among examples of what investigators said were apparently "real cases of fraud" were substitutions of healthy animals kept by IBT at the labs for animals that became ill during the tests. Investigators also found that IBT researchers failed in some cases to number test animals, making it virtually impossible to tell which were affected by the test substance.

A spokesman for IBT, a subsidisry of the Nalco Chemical Co. of Northbrook, declined yesterday to comment on the allegations. The spokesman said IBT was cooperating with the federal investigators and that the audits lof IBT's labs are still under way. IBT changed its top management last spring, after the alleged data tampering occurred.

Among the assertions of investigators, federal officials said yesterday, was that some of IBT's corporate clients apparently knew of the irregularities in the test date they got from IBT and submitted in applications for federal approval for their products.

A senior EPA official said yesterday that at least four large pesticide manufacturers knew of the alleged falsifications by IBT.

According to federal regulatory officials, the evidence that the four firms knew of the alleged data tampering turned up after EPA auditors examined only about a dozen IBT long-term animal tests. The tests are generally considered to be the base from which regulators determine the safety of potentially hazardous chemicals.

Federal auditors plan to go through about 620 similar long-term tests on 123 chemicals that IBT performed for its clients.

EPA officials say they are now considering whether to refer these cases to the Justice Department.

according to EPA officials, the agency has referred the records of one IBT client to the department for criminal investigation. Officials said the lab records of the Chevron Chemical were turned over to department investigators after it was discovered that IBT lab records covering six months of a two-year animal study on the Chevron insecticide Orthene had been falsified.

Chevron makes 8 millionm pounds of the insecticide annually. Company officials said yesterday they confirmed the falsification of the lab records on Orthene in their own investigation at IBT last year. Chevron officials said they reported the finds to EPA.

Loren Stelzer, a Chevron official, said that while the company apparently did not monitor the IBT tests closely while they were under way between 1970 and 1972, there was no attempt on the part of any Chevron employee to cover up the falsification.

Stelzer said that Chevron only has two other long-term tests on animals to determine the potential carcinogencity of Orthene. Both, he said, were potential by IBT,

EPA officials said yesterday that because of the widespread findings of test data irregularities at IBT, they plan to issue a directive next week that all firms that used the lab for applications to EPA will have to go back and recheck the raw laboratory datathey submitted.