High levels of daily stress reduce the chances of developing the disease by 40 per cent, they report in the British Medical Journal.
The finding runs counter to previous research which has suggested an increased risk of breast cancer in those under stress. Stress may also raise the risk of other diseases.
But the researchers from Denmark who conducted the latest study say sustained high levels of stress may reduce levels of oestrogen - the female hormone - which is known to affect the development of breast cancer.
The study involved 6,500 women in Copenhagen, who were followed over 18 years. They were asked at the start what stress they experienced routinely in their lives, defined as tension, nervousness, impatience, anxiety or sleeplessness.
Stress was not measured throughout the study and although the results were adjusted for factors such as whether the women had children, they did not take account of family history of the disease.
The findings showed that for every increased level on a six-point scale, there was an 8 per cent reduction in cancer risk. There were 251 women diagnosed with breast cancer during the study period, and those with the highest stress levels had the lowest risk.
"High endogenous concentrations of oestrogen are a known risk factor for breast cancer, and impairment of oestrogen synthesis induced by chronic stress may explain a lower incidence of breast cancer in women with high stress," the researchers say. They warn that stress may have other damaging effects, especially in heart disease.
A study in Sweden, conducted over 24 years and published in 2003, suggested that women who endured high stress levels ran twice the risk of developing breast cancer. But that study focused on severe stressful life events such as bereavement and divorce, whereas the Danish study focused on daily stress.
Dr Sarah Rawlings, head of policy for the Breakthrough Breast Cancer charity, said: "It's always hard to measure the impact of stress on breast cancer risk as it's difficult to untangle from other factors in our lives and everyone views their own stress levels differently. This study doesn't help us to draw further conclusions. However, maintaining a healthy balanced lifestyle is important - high stress levels can lead to unhealthy behaviour, which may alter your risk of breast cancer and other diseases."