More Americans are coming down with cancers now than ever before in history, and at least 20 percent of the cancers are associated with on-the-job exposure to hazardous chemicals, according to a major federal report issued today.

The three-year study by President Carter's Toxic Substances Strategy Committee, in conclusions already under sharp attack from industry, found that cancer among whites rose by 10 percent between 1969 and 1976 after being constant for the previous 30 years. All other causes of death declined, while a record 400,000 Americans now die of cancer every year, second only to the heart disease rate, the study said.

"This suggests some new or intensifying causal factors," said study chairman Gus Speth, chairman of the Council on Environmental Quality.

The question of whether cancer has been increasing in the American population has long been the subject of debate among scientists and doctors. Many who feel it has not contend that better detection and better treatment of other ailments that would previously have killed the sufferer are creating the appearance of an increase where none exists.However, for a group as prestigious as this to conclude the opposite is considered significant.

The causal factors are to be found in the environment, which includes diet, lift style and hormonal traits as well as workplace and overall environmental conditions, Speth said. But without ever flatly assigning blame for the cancer increase to toxic chemicals, the report called for stronger regulatory control over them and outlined ways to achieve it.

"While we don't know that toxic chemicals played a role in this development [of higher cancer rates] for certain, it is a matter of great concern to us," Speth told a news conference Friday.

Dr. Robert Harris, another CEQ member, who coordinated the report's cancer findings, said that "circumstantial evidence" linked the increase to "environmental contaminants," and that they were undoubtedly one factor.

Some of that evidence, he said, included the fact that chemical production rose dramatically between 1950 and 1960, while the cancer rate increase occurred 20 to 25 years later, "the lag time one might expect."

Other evidence involved unpublished findings by the National Cancer Institute, the National Institute for Occupational Safety and Health and the National Institute of Environmental Health Studies in 1978 that 20 to 38 percent of all cancers are associated with occupational exposure, Harris continued.

The operative words, Speth said, are "associated with," since direct causal links are difficult to establish. But the projected cancer rates among workers exposed to just six chemicals -- asbestos, arsenic, benzene, chromium, nickel and petroleum distillates -- were enough by themselves to make up 20 percent of all the estimamted 600,000 cases of cancer that occur each year, the study said.

Widening the conclusion to include other carcinogens in other jobs only shows how conservative the 20 percent figure really is, Harris said in an interview.

The American Industrial Health Council immediately attacked the study as based on questionable data. The unpublished findings in particular were unreliable, said Ronald A. Lang, council executive director.

Speth and Harris "repeatedly said that is was premature to draw conclusions . . . yet they go right on to unnecessarily alarm the public with indiscriminate charges and allegations," Land said. He called the study "inexcusable."

But the 18-agency research team defended its data as sound. One criticism, that cancer rate increases might all be attributable to lung cancer from smoking, was rebutted in detail. "Nonsmokers get lung cancer; in fact, a doubling of this disease in one decade has been reported among nonsmokers," the report said.

Most of the conclusions about overall cancer rates were based on three National Cancer Institute surveys conducted between 1937 and 1971 and an NCI Surveillance, Epidemiology and End Results (SEER) program that began in 1973.

The figures showed that, when adjusted for age and population increase, cancers rose from 346.6 per 100,000 white males in 1969 to 374.0 in 1976, and among women from 271.5 to 301.2 in the same period, a 10 percent increase.

Data for blacks were too sketchy in the early years to allow any conclusions, the study said.

The report called for new legislation and regulatory action to beef up federal control over toxic chemicals. It urged new laws to regulate cosmetics, improve access to chemical industry trade secrets and support public participation in regulatory proceedings, as well as provide funds for cleanup of spills and abandoned waste sites. A so-called "Superfund" to do that is pending in both houses of Congress.

The study said the public needs to be educated about ways cancer risks are assessed, since it does not understand that tests on laboratory animals using large test doses are valid for human cases, that most substances do not cause cancer, or that methods do not exist for determining or even defining what a "safe" level of exposure to a carcinogen might be.

"We must further strengthen federal regulation, not weaken it, if we are going to provide the degree of protection that the public demands," Speth said.