BARCELONA (Reuters) - Three or more drinks a day, whether beer, wine or spirits, boost a woman's risk of breast cancer as much as smoking a pack of cigarettes, U.S. researchers said on Thursday.

The relationship between alcohol and breast cancer is known but there has been little data on whether the choice of drink made a difference, they told a European Cancer Conference.

In what the researchers said was one of the largest studies to investigate links between breast cancer and alcohol -- found that alcohol itself and the amount a person consumed were key rather than the type of drink.

"Studies have consistently linked drinking alcohol to an increased risk of female breast cancer, but until now there has been little data, most of it conflicting, about an independent role played by the choice of beverage type," Arthur Klatsky of Kaiser Permanente in California and one of the researchers said.

Breast cancer is the second most common cancer killer of women, after lung cancer. It will be diagnosed in 1.2 million people globally this year and will kill 500,000.

Other studies have shown that light- to moderate alcohol use can protect against heart attacks, though Klatsky said other mechanisms were probably at work.

The heart protection likely comes from alcohol-induced "good" cholesterol, reduced blood clotting and decreased diabetes risk. But for breast cancer, the ethyl alcohol found in all booze likely ups the risk, the researchers said.

The researchers looked at the drinking habits of more than 70,000 women from a variety of ethnic backgrounds who supplied information during health examinations between 1978 and 1985. By 2004, nearly 3,000 of the women were diagnosed with breast cancer.

Among women who drank, the team examined a preference for a type of alcohol and how much of each drink people consumed. They also compared the total amount consumed and compared it to women who drank less than one drink a day.

Women who drank between one and two alcoholic drinks per day increased their risk of breast cancer by 10 percent compared with people who consumed less than one drink each day, the study found. The risk of breast cancer jumped by 30 percent in women who drank more than three drinks a day.

The results were also similar in different age and ethnic groups, the researchers said.