Smoking increases the risk of bladder cancer significantly more than it did decades ago, new research suggests.

Current smokers have a four times greater chance of developing the disease than non-smokers, according to the findings. This compares with an almost threefold raised risk identified by seven studies conducted between 1963 and 1987.

The greater danger for women is especially striking. Previously, 20 to 30 per cent of cases of bladder cancer in women were found to be caused by smoking. Now smoking is responsible for 52 per cent of female bladder cancers, two per cent more than the proportion of men affected by the disease.

One reason for the greater association between smoking and bladder cancer may be changes in cigarette design, say the scientists whose results appear in the Journal of the American Medical Association.

While tar and nicotine concentrations in cigarette smoke have fallen over the past 50 years, levels of specific cancer chemicals have increased.

They include beta-napthylamine, a known bladder carcinogen, according to the authors.

The new research used data from more than 450,000 participants in the NIH-AARP Diet and Health Study, a major lifestyle and health investigation in the US.