Breast cancer has overtaken lung cancer as the most common form of the disease in Britain after a steady rise in cases in the past two decades, scientists reveal today.

The 39,500 new cases of breast cancer diagnosed each year now exceed the total number of men and women who fall victim to lung cancer.

Cancer of the lung, which is now diagnosed in about 38,900 men and women a year, has been in decline because of lower rates of smoking among men. But it is still the biggest killer of all the cancers. More than 34,000 people die each year from lung cancer compared with 13,000 deaths from breast cancer, the study for two leading charities shows.

The Imperial Cancer Research Fund (ICRF) and the Cancer Research Campaign (CRC) predict that the trend will continue, reflecting changes in lifestyle choices and an ageing population.

The number of women diagnosed with breast cancer grew from 25,000 in 1979 to 31,000 in 1988, when routine screening of older women was introduced. Subsequently, the number of new cases has grown by another 25 per cent.

The rise is partly attributed to better detection of the disease. But Sir Paul Nurse, director general of the ICRF, said lifestyle factors had also contributed. "For example, more women are choosing to have their children later in life and doctors are reporting an increase in obesity, which is a risk factor in post-menopausal women."

But survival rates continued to improve, he said. Death rates from breast cancer have dropped by 30 per cent in the past 20 years and more than 70 per cent of women are now successfully treated.

By contrast, women still have high death rates from lung cancer because they are failing to give up smoking at the same rate as men.

Professor Gordon McVie, director general of the CRC, 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. There is a 'North-South divide'. Breast cancer is more common in the affluent South and lung cancer still dominates in Scotland and the far North of England."

Serious concerns about the dangers of a breast cancer drug have been raised after tests showed it greatly increased the risk of cancer of the womb lining and deep-vein thrombosis. Researchers testing the drug tamoxifen claim healthy women who use the drug to prevent breast cancer may be at high risk of contracting other diseases.

While the dangers may be high in women without the disease, the report in this week's British Journal of Cancer says tamoxifen is vital in treating women who already have breast cancer.

Dr Lesley Walker, the CRC's director of cancer information, said: "There's no doubt that tamoxifen is a vital part of breast cancer treatment but this paper suggests that it is not a magic pill which will bring an end to the disease.

"This work highlights the need to continue research to discover new treatments that tackle all aspects of the disease from prevention to treatments and cures."

Researchers from Canada used the results of the largest clinical trial of the drug to simulate the overall impact of giving tamoxifen to women at high risk of breast cancer. The results highlight serious side effects of giving the drug to healthy women over the age of 50 and indicate a possible four-fold greater risk of cancer of the womb lining and twice the risk of deep vein thrombosis.

The CRC and ICRF are part funding a trial that is due to finish next year.

Tamoxifen has been one of the greatest advances in the treatment of breast cancer and is currently given to 70 per cent of all women already diagnosed with the disease.

The report says there is no doubt that it is an invaluable treatment of breast cancer, but its use for healthy women remains uncertain.