Professor Joel Brind, from the City University of New York, and his colleagues say that although the increase in risk is low, it may account for thousands of cases of breast cancer - almost 25,000 in the US.
And they warn of a "potentially much greater impact in the next century, as the first cohort of women exposed to legal induced abortion continues to age."
The scientists, who analysed data from 28 published studies to reach an overview of the link between abortion and breast cancer, say women who are having an abortion should be told of the breast-cancer link.
They suggest that the surge in the levels of the hormone oestrogen in the first trimester of pregnancy is the most likely mechanism for increasing risk in a woman who subsequently undergoes a termination.
However, British experts reacted swiftly yesterday to allay alarm and downplay the findings of the study, published in the Journal of Epidemiology and Community Health. Margaret Gilchik, a consultant breast surgeon at St Mary's Hospital in London, said she was unaware of any link. "I am surprised", she said, "because, statistically, the person who has had a termination is more likely to have had an early pregnancy, which is a known protective factor against the disease."
Ann Furedi, director of the Birth Control Trust, said the findings should be treated with caution, and denied that the risk had been ignored by doctors.
"Even going by this paper, the risk is very small," she said. "It shows that the risk of breast cancer might be increased by about one-third after an abortion, but that has to be put in perspective. If you wait until 35 for a first pregnancy it doubles your risk of breast cancer, and if you bottle-feed instead of breast feed your baby the risk is increased by one-fifth."
Professor Brind said that the link between breast cancer and abortion was seen worldwide, which ruled out the possibility of bias or variables affecting the statistical evidence.
The first published evidence of a link appeared in 1957, but the past four decades produced neither a consensus of opinion on the issue nor "a sense of urgency to arrive at one", according to the US team.
In the US, where the breast- cancer risk is about one in eight, there are 800,000 first- time abortions a year. There are around 170,000 abortions in the United Kingdom annually, and the breast-cancer rate is 1 in 12.
t From Monday, the British Pregnancy Advisory Service will have a phone line for women worried by the report's findings, on 01564 793225.Reuse content