This week, the Government's chief medical officer called for an end to the nation's “drinking problem”, and a minimum price on alcohol to deter boozing. Wrong, said the Prime Minister, who asked: why should sensible, moderate drinkers suffer to deter the bingers?

Unfortunately for Gordon “Prudence” Brown and the rest of us, science last month provided a very good reason why. Booze, even in “moderation”, is worse for all of us than we think. A new research study told us that just one small glass of wine a day was enough to significantly raise a woman's risk of breast cancer.

This was not any old study. This was the biggest study of women's health in the world - the Million Women Study, tracking the health and lifestyles of 1.3 million women over the age of 50 in Britain.

What it found was that even low consumption of alcohol increases your risk of getting breast cancer, liver cancer, and types of bowel, throat and mouth cancer. The study indicates that just seven units of alcohol a week have a damaging effect. Above two units of alcohol a week (the amount the study used as a baseline), every daily unit of alcohol you drink increases your risk of getting breast cancer by 1.1 per cent.

In other words, one small drink every day increases the risk of getting breast cancer by the age of 75 from 9.5 in 100 to 10.6 in 100. Three units a day, and you're up to 12.8 in 100. Dr Naomi Allen, the lead author of the study, says that though the increased risk might appear small, “the high prevalence of moderate alcohol drinking among women means the proportion of cancers attributable to alcohol is an important public health issue”. One in eight cases of breast, liver and oral cancers could be down to alcohol, she says.

Is this alarmist rubbish? Not really. For years, global cancer research organisations have been convinced of the evidence of alcohol causing many kinds of cancers including breast cancer, and Cancer Research UK now asserts there is no doubt that alcohol can cause seven types of cancer and that “alcohol can increase the risk of cancer at levels far too low to make an average person drunk”. It's just a message that none of us wants to hear.

The new study sets the bar lower than previously thought. We might be jolted into taking heed this time by the very scale of the Million Women Study, a remarkable venture only possible in Britain because of its unique national breast cancer screening programme.

Between 1996 and 2001, over a million women aged 50 to 65 attending breast screening were recruited for the survey, comprising a quarter of the female population in this age group. In the ongoing survey they fill out detailed questionnaires about their lives, habits and health regularly - everything from whether they use butter or marg to how often they use the mobile phone. This information can be tied in with NHS records so the diseases they succumb to can be tracked. With such large numbers of women participating, clear patterns about which diseases are linked with which lifestyles begin to emerge.

In its short lifetime, the Million Women Study has produced persuasive evidence that women increase their risk of breast and ovarian cancer if they take the form of hormone replacement therapy combining oestrogen and progestagen, and has informed 30-odd high-quality research papers on other aspects of women's health.

There's more to come. The longer the study goes on, and the more the women develop diseases, the more those traits that are linked with the disease will become clear. As part of the research, about 60,000 of the million are providing blood samples, which provides information about their genetic and chemical make-up. When tied in with information about the diseases these women go on to develop, vital clues will be thrown up about how to spot disease early through simple blood tests.

The study, being conducted by the University of Oxford's highly respected Cancer Epidemiology Unit, isn't without its limitations. Strictly speaking, its findings apply only to middle-aged women. Certain types of people might also be more likely to volunteer for such studies, which might make the sample unrepresentative. Since most of the information is “self-reported” rather than collected objectively, can we really believe what people say about themselves? Critics point out that women embarrassed about their alcohol consumption are likely to write down that they are drinking significantly less than they really are.

And could the rise in cancers among women who drink be attributable not to alcohol itself but some other characteristic associated with people who drink, that they generally live less healthy lives than non-drinkers for example? It's possible, but the researchers have statistically weighted their findings to take account of such possible confounding factors. If we accept the Million Women findings, by what possible physical mechanism might one glass of wine a day increase your risk of cancer?

According to Cancer Research UK, it is likely to work differently for different cancers. For women's cancers, such as breast and ovarian cancer, high levels of sex hormones such as oestrogen have long been known to increase risk. Studies have already indicated that blood alcohol can encourage the production of these hormones.

In the liver, alcohol damages cells, making them more vulnerable to cancer. In the mouth and throat, alcohol makes it easier for the tissues to absorb the cancer-causing chemicals in tobacco. And there may be a general effect too; a chemical produced by alcohol called acetaldehyde is known to cause cancer by damaging DNA.

Whatever the merits of the Million Women Study it remains one study. Should it be enough to stop us drinking? It depends on how high a priority you put on maintaining your physical health, and how many social and psychological benefits you associate with drinking.

View it in the context of those studies we've read about indicating that moderate drinking may have a role in preventing heart disease and stroke, then there seems to be a health balancing act to perform. But it may not be an equal equation. The Journal of the National Cancer Institute in America published an editorial to accompany the Million Women report on alcohol and cancer stating that the increased cancer risk appeared to outweigh any cardiovascular benefits.

Also on the minus side of the health equation is the astonishing statistic that in developed countries, cirrhosis (usually caused by alcohol) is the third most common cause of death in people aged 45 to 65. And if you thought that was just the down and outs and bingers, you'd be wrong. According to the Centre for Public Health at Liverpool John Moores University, more than a quarter of adults in the most prosperous parts of the country are drinking at “hazardous” levels.

But if this study is to be taken seriously, there's an unpalatable truth for middle England. We cling to moderation in all things, but the fact is that, where our health is concerned, most of us have a drinking problem. The Million Women study doesn't mean we have to stop, but perhaps it does mean that we can't ignore the facts any longer.

“I think research like this is unpopular because people find it difficult when the safe limits aren't clear,” says Dr Lesley Walker, director of information for Cancer Research UK, which provides funding for the Million Women Study. “There's the Government safety limits of 21 units a week for men, and 14 for women, and then we're saying that if you're a woman and you want to minimise risk, you probably shouldn't drink anything.

“You still have to remember that the risk of one small drink a night is small,” she says. “We're not putting out a message that people have to stop. But the clear line emerging is that the less you drink, the less likely you are to get cancer.”