The study is the first to demonstrate a clear link between increased liquid intake and decreased bladder cancer.

Although water had the most powerful effect, drinking plenty of any liquid, including beer, was beneficial.

However, experts warn that drinking too much alcohol is bad for your health.

Quantity not quality

Dominique Michaud, a research fellow at the Harvard School of Public Health, led the study, which is published in the New England Journal of Medicine.

The team found that men who drank at least 11 eight-ounce glasses a day of all liquids cut their risk of developing two common types of bladder cancer in half, compared with men who drank five glasses or less.

Even smokers who cannot or will not quit can still cut their cancer risk substantially by drinking more, Dr Michaud said.

Bladder cancer strikes an estimated 310,000 people worldwide each year.

It is more likely to affect smokers, and men are four times more likely to develop the disease than women are.

Study of habits

The Harvard study examined the eating, drinking, exercise and smoking habits of 47,909 American men from 1986 to early 1996.

A similar large-scale study of women does not include enough cases of bladder cancer yet to yield statistically significant results, Dr Michaud said.

Of the men, 252 developed bladder cancer. The risk increased with age.

When the researchers looked specifically at water, they found that men who drank at least six glasses a day were half as likely to develop the cancer as men who drank less than one.

This was unaffected by the total amount of liquid consumed.

Possible causes

There is no clear explanation as to why liquids should reduce the cancer risk.

Some researchers think it could be because the bladder lining suffers less exposure to cancer-causing substances in urine when the urine is diluted and urination is more frequent.

However, Peter Jones, director of the University of Southern California's Norris Comprehensive Cancer Centre, sounded a note of caution.

In an accompanying editorial, he said the study had not been sensitive enough to determine whether water quality - especially chlorination - played a role in cancer risk.