Women who have an abortion or miscarriage are at no greater risk of developing breast cancer, says a study in The Lancet.
Abortion and miscarriage do not affect a woman's chances of developing breast cancer
In the largest study of its kind, researchers at the University of Oxford analysed data from 53 studies, looking at 83,000 women with breast cancer.
Previous studies have suggested an abortion can increase the risk of developing breast cancer later in life.
But researchers say these studies used unreliable data and research methods.
The latest research backs a Swedish study published earlier this year that found women who had an abortion were no more likely to develop breast cancer.
Giving birth to a child at full-term is known to reduce the risk of breast cancer.
Although scientists are unsure why, it is suggested that the hormones triggered by pregnancy may have a protective effect.
But some have questioned whether women who terminate a pregnancy could lose this 'protection', putting themselves at a higher risk of developing breast cancer.
Professor Joel Brind, from the City University of New York, said earlier this year that most research indicates this to be the case.
He said 29 out of 40 studies from around the world analysed by his research team showed women who terminated a pregnancy were at higher risk of breast cancer.
However Professor Sir Richard Peto, from the Cancer Research UK Epidemiology Unit in Oxford, said: "Some previous reviews on abortion and breast cancer have reached mistaken conclusions because they mixed together data from reliable and unreliable types of study.
"This is the first time that so much information has been brought together and the findings are more reliable than ever before."
Professor Valerie Beral and colleagues found out of the 83,000 women, only 44,000 had predicated in reliable studies.
They were asked before they were diagnosed with cancer to tell researchers whether or not they had an abortion or miscarriage.
Results from these women showed that for those who had miscarried there was no increased risk of breast cancer compared to the general population.
The risk was actually slightly lower among those who had an abortion.
Professor Beral told BBC News Online: "The results should make it clear that studies which are properly designed show pregnancies that end in abortion don't increase the risk of breast cancer."
She said previous studies have never been based on any biological evidence.
Delyth Morgan, chief executive of Breakthrough Breast Cancer, said: "We hope that this research will put a stop, once and for all, to the persistent claims that abortion is a risk factor for the disease and give reassurance to women."
She said concerns about breast cancer should not influence a woman's decision whether or not to have an abortion.
Ann Furedi, chief executive of the British Pregnancy Advisory Service, an abortion provider, said: "The incorrect claims of links between abortion and breast cancer... have caused unnecessary anxiety to women with unwanted pregnancies and unnecessary distress to former abortion patients who have coincidently been diagnosed with breast cancer later in life."
LIFE chairman Professor Jack Scarisbrick said it is misleading of BPAS to claim this study resolves the issue.
"Almost every study into breast cancer cites high levels of oestrogen as a big part of the answer," he said.
"Never is oestrogen at such high levels as in early pregnancy when it increases by 2000% and triggers the multiplication of breast cells.
"If the pregnancy is artificially cut short as in induced abortion, these cells are left highly vulnerable to carcinogens."