A number of lifestyle factors may also be contributing to breast cancer trends

Sir Paul Nurse

Experts predict that the gap between breast and lung cancer will widen still further.

The shift is remarkable since breast cancer occurs almost entirely in women whereas lung cancer affects an estimated 38,900 men and women.

Experts know that the overall decline in the number of new lung cancer cases reflects the falling number of male smokers over the last 50 years.

Baffled

Possible reasons for increased risk

Delayed childbirth Lack of exercise Increasing obesity levels Poor diet with too few vegetables Increased consumption of dairy products

However, they admit they are still somewhat baffled by the steady rise in breast cancer rates over the last three decades, only part of which can be explained by improvements to screening programmes.

Sir Paul Nurse, Director General of Imperial Cancer Research Fund, said: "The good news is that, despite the increase in the number of new breast cancer cases, survival rates continue to improve and over 70% of women are now successfully treated.

"A number of lifestyle factors may also be contributing to breast cancer trends.

"For example, more British women are choosing to have their children later in life and doctors are reporting an increase in obesity - in post-menopausal women this is a risk factor."

Breast cancer, like most cancers, is largely a disease of older age, with around 80% occurring in post-menopausal women.

Lung cancer, however, is generally a consequence of smoking, with up to 90% of cases related to tobacco.

Female smokers

Although the number of people diagnosed with lung cancer is falling, this masks the fact that cases are continuing to rise among women.

Professor Gordon McVie, director general of the Cancer Research Campaign, said: "Women didn't take up smoking in large numbers until the 1960s and, because lung cancer takes 20 to 30 years to develop, we're only now seeing its deadly impact.

"The figures for these two cancers mask a North-South divide with breast cancer being more common in the affluent South and lung cancer still dominating in Scotland and the far North of England.

"I wish I could offer a magic formula for preventing breast cancer, but unfortunately there isn't one.

"However, by being breast aware and going for screening, it's more likely that the disease will be detected early when there is a 90% chance of survival.

"Smokers, on the other hand, have it entirely within their power to reduce the lung cancer toll."

Delyth Morgan said the reasons for the increase were not fully understood

Delyth Morgan, chief executive of the charity Breakthrough Breast Cancer charity, said there were a number of factors affecting the likelihood of women developing breast cancer.

She said: "Women's lives now are so different to how they used to be.

"Two generations ago my grandmother had 12 children, women aren't going to start having 12 children now.

"But there are other factors too. The whole 21st century lifestyle is very different, and I don't think scientists understand properly what all the factors are in there conspiring to make this increase happen."