Physical activity levels are not influenced by breast size, which suggests they could be a barrier to exercise for women across all bra sizes, the survey by Portsmouth University's Research Group in Breast Health found.
Researcher Emma Burnett found that breasts were ranked fourth as a barrier to exercise after lack of energy, time constraints and health reasons.
They were ranked above other barriers including cost of exercise, access to facilities, not having the right clothing, not having any company while exercising, and feeling embarrassed about doing sport.
The research - published online in the Journal Of Physical Activity And Health - found that 17% of the 239 women surveyed were discouraged from participating in physical activity because of their breasts.
The most influential factors preventing women taking exercise include not being able to find the right sports bra and being embarrassed by excessive breast movement.
Miss Burnett said: "This latest research is crucial because it shows that breasts are an important factor to consider when investigating what deters women from exercising. One third of women in the study were not meeting physical activity guidelines, but improving breast health awareness may help to encourage participation for these women.
"We had plenty of anecdotal evidence to suggest that breasts are a barrier to exercise but this is the first study to provide evidence that they are affecting women's participation in sport."
Miss Burnett said she hoped manufacturers to take note that some women struggle to find the right sports bra.