Young women who take birth control pills for more than four years run a significantly higher risk of breast cancer, according to a study published Friday in a prestigious British medical journal.

Researchers were quoted in The Lancet as saying that among women 35 or younger, they found a 43 percent increase in the risk of breast cancer after four years of pill use and a 74 percent increase after eight years.They called on doctors to tell women about the possible risks of breast cancer before they prescribe contraceptive pills, but they did not advocate avoiding oral contraceptives.

However, Clair Chilvers of the Institute of Cancer Research in London, one of the authors of the study, advised women to aim for the lowest-dose pill available and stay on it for the shortest possible time.

"Women who are not having intercourse should stop taking it," she said Thursday at a London news conference.

The study is the most comprehensive look at the pill and breast cancer ever undertaken in Britain and one of the largest in the world.

The researchers were quoted in the medical weekly as saying the risks of taking the pill had to be kept in perspective.

"Breast cancer is uncommon below age 36, the age group that was studied. Only one woman in 500 develops the disease before age 36, so even a 70 percent increase in risk would only put the chances of developing breast cancer by this age up to about one in 300," the researchers said.

Earlier studies on the relationship between the pill and breast cancer have conflicted. Last January, a U.S. Food and Drug Administration committee of experts declared that recent research on possible links between the pill and breast cancer was inconclusive.

The British study involved 755 British women under age 36 who were diagnosed as having breast cancer between the beginning of 1982 and the end of 1985, and an equal number of similar but cancer-free women.

The researchers, from Oxford University and two leading British cancer charities, said they still could not answer the "crucial question" of whether the increased risk persisted after age 36. They said they are undertaking a study on women ages 36 to 45 "to help resolve this vital issue."

"The most consistent interpretation of our results and those from previous studies is perhaps that the excess risk is transient and diminishes or disappears at older ages," they said.

The researchers said their study included only those women with breast cancer who had the opportunity to use oral contraceptives from the start of their reproductive lives. Since the pill has been available in Britain since the early 1960s, they decided to include only women age 36 and under at the time their breast cancer was diagnosed.

The researchers said 91 percent of the women with breast cancer and 89 percent of the comparison group had used the pill at some time.