Federal epidemiologists said yesterday that results of a new study virtually clear the birth control pill as a factor in causing breast cancer in women under 45. But the suggestion that the study closes the book on the link between breast cancer and the pill was quickly disputed by other government cancer specialists.

Even Lancet, the British medical journal that is publishing the study in its current issue, states editorially that "firmer conclusions must perforce await more epidemiological investigation."

The study by the National Institute of Child Health and Human Development surveyed pill use in about 2,000 young breast cancer patients and 2,000 healthy women of the same ages. Dr. Bruce V. Stadel, the NICHD medical officer who headed the study, said that it "correctly describes pill use in this country as having had no overall impact on the risk of breast cancer in women under 45." Because other methods of contraception -- specifically the intrauterine device -- have been linked to serious side effects such as infertility, Stadel said the new study "on balance leads toward the suggestion that the pill" may be a better choice for many young women.

Two earlier studies, one done in England, the other in Los Angeles, both published in 1983, suggested a strong link between long-term use of the pill by women under 25 or before their first pregnancy and the incidence of breast cancer at a relatively early age. Pill users were three to more than four times more likely to get breast cancer.

The Lancet editorial suggests that the discrepancy may be partially explained by the fact that pill use among young women became widespread a few years earlier in England than in the United States; and in California before other parts of the United States.

National Cancer Institute epidemiologist Louise Brinton said, "Our major concern is in the numbers of young women who were using the pill for an extended time. I believe they in the NICHD study had limited numbers in that category and that there is a continued need for surveillance among this subgroup."

Pill use began around 1960, but became widespread only in the 1970s. Lancet estimates an approximate five-year lag between the pill's growth in popularity in England and its widespread use in this country.

Brinton said she was also concerned at the suggestion that the new figures were to be considered definitive.

"Not because we consider the study flawed in any way," she said, "but . . . that nothing is definitive. It does not confirm the two previous studies showing excess risk, so that it is something on the side of negativity. But more surveillance is needed."

Dr. Sidney Wolfe, director of the Public Health Citizen Research Group, said that the new study "gives women a false sense of security . . . . There are now studies indicating positive links and negative links. My advice is to go with the cautious position and say it is possible the pill may have an impact."

The breast cancer study is part of a larger examination of the relationship of oral contraceptives and reproductive cancers. Results have already suggested that the pill has a protective effect against uterine, endometrial and ovarian cancers, and initial evidence that the pill may play a role in causing cervical cancer is being reviewed.