A new National Academy of Sciences report questions the widely held assumption that switching to cigarettes with less tar and nicotine necessarily will reduce the health risks of smoking.

In a report released yesterday, an 18-member academy panel said that the available scientific evidence to support such a switch is "doubtful."

The committee concluded that the only sure way to reduce the risk of cancer and heart disease associated with smoking is to "quit smoking entirely."

The report noted that a new analysis conducted for the committee found a "substantial and unexpected" increase in lung cancer deaths over a 20-year period in which the amount of tar and nicotine in cigarettes was cut in half.

This finding was based on estimates of the total number of cigarettes smoked by American men during their lifetimes and men who died from lung cancer in 1955-75.

"We are left with the conclusion that, at least through 1975, cigarettes had not become appreciably less hazardous for men" between the ages of 35 and 65, the panel said. Similiar data for women were not available.

Committee Chairman Louis Lasagna, of the University of Rochester, said in an interview yesterday that the new assessment of the benefits of low-tar-and-nicotine cigarettes was the most pessimistic to date by an independent health group.

"The committee started off with the impression that this was an alternative to stopping smoking, but as we looked at the data we did indeed begin to get gloomy about just how much has been accomplished," he said.

The reasons for the surprising increase in lung cancer deaths over that time are unknown, but the panel suggested several possibilities.

One concern is recent evidence that those who switch to the low-tar cigarettes may start smoking more cigarettes or inhaling more smoke. There are also other hazardous subtances in cigarettes, such as carbon monoxide or hydrogen cyanide, that might offset the possible benefits of the switch, the committee said.

But the group noted that it may be too early to see the benefits because of the long time it takes to develop cancer after exposure.

A spokesman for the industry trade organization, the Tobacco Institute, contended that the report provided "no new data" and was "at odds" with an American Cancer Society study that found "substantial advantages of low-yield cigarettes." He said, however, that the industry had "never maintained there is a health benefit from switching to lower-tar-and-nicotine cigarettes."

But the author of the cancer society study, Dr. Lawrence Garfinkel, responded that, while his study had found some benefits, they were "minimal." "We say that the best thing is not to smoke. For people who can't stop, the evidence from our study suggests that switching to a low-tar-and-nicotine cigarette may be less harmful." A Public Health Service spokesman offered similar advice.