A long-awaited study by the National Academy of Sciences yesterday warned that saccharin is clearly a potential cancer-causing agent when used by humans, though one of low potency compared with some other substances.

The report, ordered by Congress last year, said saccharin may be particularly dangerous to young children, who consume it heavily and over long periods of time in soft drinks, where it is used as a low-calorie substitute for sugar.

Saccharin, available as a sugar substitute for more than 70 years, is ingested by 50 million to 70 million Americans, including a third of all children under 10 and 80 percent of all diabetics, the NAS said. Cyclamates, the only other major artificial sweetner, were outlawed in 1970 as cancer-causing.

Yesterday's report doesn't mean that saccharin will be taken off the market. A second NAS report, due Feb. 1, is studying whether to bar saccharin altogether, restrict its use or do nothing.

Saccharin has been the subject of bitter controversy for years. Last year, the Food and Drug Administration, on the basis of tests made on rats, proposed to ban it. Under the "Delaney amendment" of the food additives law, any substance causing cancer in rats must be banned.

But protests from the soft-drink industry and an outburst of public opposition led Congress to pass a law blocking the FDA from taking any action until May 23, 1979.

The law also ordered the two NAS studies.

Part of the public furor last year against the proposed ban arose from the fact that the doses fed to the laboratory rats were extremely heavy - far in excess of those normally ingested by a human.

However, the NAS letter transmitting yesterday's report to the Department of Health, Education and Welfare said that, "Because of the great size of the American population and the number of saccharin users, even a tiny risk can be calculated to result in" anywhere from none to 3,000 additional cases of bladder tumors in American males annually.

The NAS panel included leading figures from medical schools and research facilities. It reviewed existing studies of saccharin effects on rats and reported these conclusions yesterday:

"Saccharin must be viewed as a potential carcinogen (cancer-causing substance) in humans, but one of low potency in comparison to other carcinogens. Although saccharin would be expected to be of low potency in humans, even low risks, applied to a large number of exposed persons, may lead to public health concerns."

In rats, saccharin causes cancer not only acting by itself, but it also "promotes the cancer-causing effects of some other carcinogenic compounds." This second effect, as an enhancer of the cancer-causing effects of those substances that may be in the body may actually pose the greater danger of cancer.

It is the saccharin itself, and not any impurities in it, as claimed by some, that causes cancer.

It is not possible to predict from the rat studies just how powerful a cancer-causing agent saccharin is in humans.

No further laboratory tests are needed on the question of whether saccharin is a cancer-causing substance. That is established. However, it would be useful to have further studies on just how it works and on several other related questions.

"Essentially, there is no scientific support "for the contention that there are health benefits from saccharin. Although saccharin clearly is usable as a substitute for sugar for persons with overweight and diabetic conditions, "data available . . . do not permit the evaluation (on a long-term basis) of the efficacy of saccharin the control of disbetes or body weight" or a determination of whether saccharin has a major effect in decreasing dental decay when used as a substitute for sugar.

Discussing saccharin's role as an enhancer of cancer-causing effects of other substances, the panel said that "When administered to animals that had been previously, exposed to low doses of some other bladder carcinogens, it increase the incidence of bladder cancer in both" male and female rats.

As a direct cause of cancer - an "initator of cancer" - saccharin has been shown to cause cancer in male rats when the mother is exposed to saccharin before pregnancy, the fetus is exposed to it during the pregnancy period and then the offspring is exposed to it.

This suggests, the panel indicated, that saccharin could be a danger if used by pregnant women and the child then grew up using it too.

FDA Commissioner Donald Kennedy said the study by NAS confirms the FDA's conclusion that saccharin is a carcinogen. He added, "It is particularly significant that the NAS scientists expressed concern about the exposure of children and women of child-bearing age to saccharin and concluded that there are no demonstrated benefits from the use of this artificial sweetener."