Stockholm: Stress at work can increase the risk of women developing breast cancer by upto a third, a new study of Swedish women suggests.

It is thought the increased risk is a result of poor lifestyle choices which lead to obesity, increased alcohol consumption, leading to a weakened immune system.

The study published in the journal of Epidemology found that women in demanding jobs are 30 per cent more likely to develop the disease than those who feel on top of their work.

The results of the study, which involved 36,000 women, appear to contradict previous research which has not found a link between stress and breast cancer.

Thousands of women die each year from the disease and although survival rates are improving, the number contracting it is on the increase.

The study looked at information on 36,000 Swedish women aged 30 to 50 who were in work when the study started in 1990. The study followed the women until 2004, by which time 767 of them had been diagnosed-with breast cancer. The risk of breast cancer increased by around 30 per cent for women with stressful jobs after other factors, such as alcohol consumption, number of children, weight, and age, were taken into account.

The Swedish researchers found no link between stress and cancer among women in part-time work.

The reason why stress might increase the risk is unclear, although studies show it may raise levels of the hormone oestrogen which can heighten the risk of cancer. Another theory is that stress changes women’s behaviour, making them adopt unhealthy habits such as smoking and not exercising.

Recent research has found that long working hours and stress from work can bring on the menopause early and, in pregnant women, increase the risk of a miscarriage.

However, a Danish study of 7,000 women over 18 years found those with high levels of stress were less likely to develop breast cancer than women with low stress levels. Leading-cancer scientists yesterday said more research is needed before stress can join other well-known risk factors.